Fish pond design, construction, equipment, fish, plants, algae Control & maintenance.

Site last updated 14 November, 2016


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Fishpond Solutions is primarily an information site for water gardens and hobby aquaculture. Fish pond water quality is the theme of this site. Pond design, construction and maintenance directly influence water quality. The fish pond books and e-documents offered here cover all aspects of ponds.

The general principles for fish ponds are similar everywhere, but the warmer the climate, the worse the ponds' water quality problems and the more effort needed for algae control.

My name is Lloyd Mathews. I have Bachelor of Science degrees in aquaculture and environmental biology (the latter with Honours) from Curtin University, Western Australia. I ran a fishpond maintenance business for seven years. Since then, I have been slowly building a fish hatchery and aquaponics set-up on my home property. I also teach science, part-time, in high schools. As well as science degrees, I have a diploma in freelance journalism.




Fish ageing (14 November, 2016)

H ow do you tell a fish's age? You can try asking it, but if it won't tell you, look at its otoliths (calcified ear bones). Some fish, however, have gelatinous otoliths which can't be used for telling their age. One such fish is the Australian Lungfish, a living fossil that has been around for a hundred million years. A new method for ageing the fish has been discovered. Testing nuclear bombs increases the level of one of the isotopes of carbon (carbon -14) in the atmosphere and oceans. Extensive testing of British nuclear weapons in Australia in the 1950s and 1960s has left a distinctive carbon-14 signature in the tissues of organisms from that period. The signature from the scales of lungfish from that period has been successfully used to age the fish.

The above information is from an article written by Natasha Prokop for "Fish", a news magazine put out by the Fisheries Research and Development Corporation in Australia.

Water shortage from Global Warming (3 September, 2013)

Where I live, rainfall has decreased by about 40% over the last 20 years - which was the decrease predicted 20 years ago. Does that mean we should do away with garden ponds in order to save water? The amount of water used in summer to top up a pond to replace the water lost by evaporation is probably no more than the amount used on any other part of the garden. So unless you are going to replace your water-hungry garden plants (those of foreign origin) with native plants, I don't see the point of worrying about filling in your pond. If you want to keep your pond and save water, you can set up a hydroponics garden and incorporate it with your pond (called "aquaponics").

Aquaponics (3 September, 2013)

Combining hydroponics with aquaculture is becoming very popular here in Australia. Called aquaponics, it is a match made in heaven - the excrement and excretions of the fish fertilize plants: the plants purify the water for the fish. All you usually need to do is feed the fish. If your water is very "soft", like rainwater, you might need to add calcium but you don't need to add expensive hydroponics additives to make your plants flourish. Another big plus is that hydroponics uses much less water than earth gardens and the addition of the fish takes very little additional water.

Global Warming - is it our fault? (3 September, 2013)

The truth is out there, but finding it on the Internet is difficult to say the least. All credible government agencies including NASA and Australia's CSIRO blame human activity entirely for the current warming of the globe. These agencies take into account all evidence from all scientific sources. If you search the net, however, you are bombarded with sites claiming that global warming has nothing to do with us.

According to these sites, Mother Nature ignores trivia such as the tearing down of most of the world's forests and, at the same time, digging up millions of years of stored fossil fuels, burning them and sending the carbon into the atmosphere. They back up their claims by taking one set of data from one scientific discipline which doesn't seem to agree with all the other evidence. Greenpeace has found connections between many of these websites and oil companies. Surprise, horror - they wouldn't do that, would they?

Personally, I have no reason to doubt agencies like NASA. After all, they were competent enough to put a man on the moon, weren't they? Or did they? That's a scam too, according to some people. If I believe that most scientists are incompetent and liars, then I'll believe that the world is flat. After all, look out your window - you can see it's flat! If you don't believe that global warming is our fault, you should join "the earth is flat" society. They welcome people like you. I might join too - I like a good argument.

The sad part about global warming is that if it really is our fault and we continue to do very little about it, we will leave a legacy of misery to our children and grandchildren.


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The Complete Practical Fishpond Book

My aim in this book is to give essential advice on all the main aspects of freshwater garden fish ponds. I have tried to include some of the science of ponds while keeping the information easy to understand. Each chapter begins with an outline of the main points of the topic. Each point is then expanded on.

My experience with ponds has mainly been in the warm temperate climate of Perth, Western Australia. The principles for fish ponds are similar worldwide but warm temperate climates like that of Perth intensify some of the problems in ponds. This book will therefore be particularly useful to pond owners in sunny climates.

The book begins with pond design, starting with the position of the pond in the garden. I discuss the consequences of different pond sizes and depths and of “natural” and “artificial” ponds. I talk about the advantages of a dual pond system. I give guidelines for these and for “self-cleaning” ponds. I also say why I recommend designing the pond with a sump, overflow, leaf skimmer, and automatic top-up valve. Lastly, I give my colour preference for the pond bottom and sides.  

Next, I advise on pond construction. Ponds can be built with concrete, bricks, rigid polyethylene, fibreglass or liners. I write about my experiences with ponds made from each of these materials and also my preferences for pipework materials.

In the following chapter, I recommend various pumps, filters (including ultraviolet clarifiers), water features, underwater lights and copper ionizers. Choosing the right equipment will give you the right effect for the lowest cost and for the least effort.

The chapter on fish gives information on types of fish, especially goldfish and koi. I advise on when a new pond is ready for fish and the number of fish a pond can support. I give information on the handling, transporting, and feeding of fish, and on diseases and predators.

The next chapter delivers general information on water plants – why you should have them and their role in the ecology of the pond. Plants provide shade, oxygen, food, habitat and cover from predators. They filter toxins and excessive nutrients from the water. I give advice on keeping plants, including information on fertilizer and pests.

Finally, I give recommendations for pond maintenance including a routine. My advice is directed at pumps, filters, pond cleanliness, exchanging water and maintaining the pH and hardness. The maintenance is largely directed at algae control. I discuss the various forms of nuisance algae and control methods control for microalgae, blanket weed, and slime algae. Other advice includes information on water testing and water treatments.  
Maintaining good water quality is fundamental to the success of any fish pond. "Good" water quality means the water's suitability for its proposed purpose. Water quality is affected by every aspect of a pond, from its design and construction to its pumps, filters and maintenance. Each chapter of this book tells how each aspect of the pond affects the water. Every fish pond is different. The solutions to one pond's problems may be very different to another pond's.

© Lloyd Mathews BSc(Hons) 2012

The PDF version of this book is 123 pages including a glossary of technical terms, numerous photos and other illustrations. Numerous links throughout the book expedite navigation around the book and to the glossary.

$9.99 Add to Cart  View Cart

To buy the printed book from Amazon's "Createspace", click on the following:


Algae Prevention and Control

The most common problem with fish ponds is the excessive growth of unsightly algae. This e-book tells how to prevent algae growing in ponds and thirteen ways to get rid of algae.

© Lloyd Mathews BSc(Hons) 2012

The PDF version of this document is 21 pages including a glossary of technical terms and several photos. Numerous links throughout the document expedite navigation around the document and to the glossary.

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Fish Pond Design

"Fishpond Design" is the first chapter of my book "The Complete Practical Fishpond Book". It begins with the best position of a pond in the garden. This chapter discusses different pond sizes and depths, and natural and artificial ponds. It talks about the advantages of a dual pond system and gives guidelines for self-cleaning ponds. It also recommends designing the pond witha sump, overflow, leaf skimmer, and automatic top-up valve. Lastly, it gives give my colour preferences for ponds.

© Lloyd Mathews BSc(Hons) 2012

The PDF version of this document is 18 pages including a glossary of technical terms and several photos. Numerous links throughout the document expedite navigation around the document and to the glossary.

$3.99 Add to Cart  View Cart


Fishpond Construction

This is Chapter 2 from my book "The complete Practical Fishpond Book". It gives advice on building ponds with concrete, bricks, rigid polyethylene, fibreglass and liners. I tell of my experience with these materials. I also give my preferences for pipework materials.

© Lloyd Mathews BSc(Hons) 2012

The PDF version of this document is 10 pages including several photos. Links throughout the document expedite navigation around the document.

$2.99 Add to Cart  View Cart


Fish Pond Equipment

This document is the third chapter from "The Complete Practical Fishpond Book". As well as all types of pumps, filters and water features, it discusses leaf skimmers, copper ionizers and underwater lights.

© Lloyd Mathews BSc(Hons) 2012

The PDF version of this document is 23 pages including a glossary of technical terms and numerous photos and other illustrations. Numerous links throughout the document expedite navigation around the document and to the glossary.

$3.99 Add to Cart  View Cart


Pond Fish

This document is the fourth chapter from my book "The Complete Practical Fishpond Book". It asks whether you need fish in your pond, how many fish can your pond hold, and what kind of fish should you have. It tells how to handle, transport and feed fish. When and how to put the fish in the pond is also discussed. Information is given on fish diseases and their treatment.

© Lloyd Mathews BSc(Hons) 2012

The PDF version of this document is 34 pages including a glossary of technical terms, tables, photos, and other illustrations. Numerous links throughout the document expedite navigation around the document and to the glossary.

$3.99 Add to Cart  View Cart


Pond Plants

This document is the fifth chapter from "The Complete Practical Fishpond Book". It tells why it is important for plants to shade the water and filter nutrients and contaminants. Other advice includes keeping plants in pots and not fertilizing. Plant types and plant pests are discussed.

© Lloyd Mathews BSc(Hons) 2012

The PDF version of this document is 15 pages including a glossary of technical terms and several photos. Numerous links throughout the document expedite navigation around the document and to the glossary.

$2.99 Add to Cart  View Cart


Fishpond Maintenance

This document is the sixth and last chapter from the book "The Complete Practical Fishpond Book". Apart from maintaining pumps and filters, this chapter tells how to keep the pond water clean using a maintenance routine. Lastly, it advises on algae prevention and control.

© Lloyd Mathews BSc(Hons) 2012

The PDF version of this document is 27 pages including a glossary of technical terms, photos and illustrations. Numerous links throughout the document expedite navigation around the document and to the glossary.

$3.99 Add to Cart 

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Water Quality

Good water quality is the most important thing with any fish pond. "Desirable " is probably a better term to use than "good" because different ponds require different levels of quality. If you want to see your koi in all their glory, you will probably want crystal clear water devoid of plants. If, however, you want a pond filled with glorious flowering plants, you can't have the pure, sterile water of a chlorinated water feature. 

Sunlight, aeration, and phosphorus are usually the most important factors affecting fish pond water quality. Phosphorus and sunlight are the main factors limiting plant and algae growth. Aeration not only makes oxygen available to fish, it also retards the development of the types of bacteria that produce noxious odours.

The less sunlight a pond receives, the less algae grows. If a pond is in the sun for too much of the day, phosphorus control is usually the best way to keep algae in check.

Unless a pond is very heavily stocked with fish, a suitably sized pump running continuously will provide adequate aeration. The most important time to run the pump is during the hours of darkness, especially during the hours just before dawn. At night, plants and algae don't produce oxygen. The living things in the pond, including the plants, algae and most bacteria continue to use the oxygen causing the level of dissolved oxygen in the water to drop during the night. If, however, your pond has more than a few fish, the pump will need to run continuously to bring a constant supply of oxygen and ammonia to the nitrifying bacteria in the pond. Without the nitrifying bacteria, ammonia and nitrite could reach levels which are toxic to fish.


Pond Terms Definitions

Aeration: Adding oxygen to water by bringing air into contact with the water.

Aerobic: Relating to or caused by the absence of oxygen. (For living things: needing oxygen to be active or living.)

Algae: Members of several phyla of the kingdom called Protista. Although algae have much in common with plants, they also have many differences. Algae members include the common nuisance algae of ponds - microalgae, hair algae and slime algae.

Alkalinity: The total concentration of alkaline substances in the water expressed as equivalent calcium carbonate. In fresh water, alkalinity and hardness are usually similar because most of the alkalinity normally comes from calcium and magnesium carbonates.

Amphibian: A member of the class of animals that includes frogs, toads, newts and salamanders.

Ammonia: A compound of nitrogen and hydrogen which is highly soluble in water and is toxic to fish at extremely low concentrations. Decaying organic matter (e.g. leaves, algae, fish excrement) produces ammonia in ponds. 

Anaerobic: Relating to or caused by the absence of oxygen. (For living things: able to live in an environment which has no free oxygen.)

Aquaculture: The growing of animals and crops in water (underwater farming).

Aquaponics: The combination of hydroponics and aquaculture. Fish are fed in tanks and the water is circulated through a hydroponic system to supply nutrients to the plants in the form of fish excretions (fish fertilizer). The plants absorb the nutrients (nitrates in particular) and the purified water is then circulated back to the fish.

Blanket weed:  see hair algae.

Biological filter: Any surface which harbours nitrifying bacteria.

Denitrifying bacteria: Bacteria which convert nitrate to nitrogen  (some aquaculture engineers mistakenly use this term when they are talking about nitrifying bacteria).

Dissolved oxygen: Oxygen dissolved in water making it available for respiration for underwater organisms (e.g. through the gills of fish).

Ecosystem: A community of interacting organisms together with the environment with which they also interact (e.g. a pond).

Free ammonia: That part of the ammonia in water which is not bound up as harmless ammonium ion. ("Free" ammonia and ammonium ions exist in an equilibrium which varies with changes in the water's temperature and pH. It is the free ammonia which is highly toxic to fish.)

Genera: Plural of "genus", a term which classifies a group of closely related species.

Green water: Water turned green by microalgae.

Hair algae: A common name for several closely related genera of filamentous green algae. (Also called string algae and blanket weed. In fresh water, the common genera are Spirogyra, Mougeotia and Zygnema.)

Hardness: The amount of dissolved minerals in water. In fresh water, hardness usually consists mainly of the total amount of calcium and magnesium dissolved in the water. It is expressed as milligrams per litre of equivalent calcium carbonate.

Head: The height above the pond's surface to which a pump delivers water.

Heavy metal: A metal with a density greater than five times that of water. The term includes iron, copper, zinc, cadmium, mercury and lead. Heavy metals can accumulate in animal tissues, including those of fish, until they reach toxic levels.

Impellor: The part of a pump which pushes water through the pump housing.

Kingdom: One of the major groups of organisms into which living things are classified.

Liner: Any waterproof material used to line a pond.

Magnetic drive pump: A pump which operates by using an electromagnet to turn the pump's impellor instead of using a drive shaft. (The advantage of this type of pump is that there are no moving parts between the motor and the impellor so the pump lasts much longer than other pumps.)

Maximum water flow: The flow a pump gives when the water's head is zero.

Mechanical filter: Any filter which physically removes suspended particles from water.

Microalgae: Microscopic forms of algae (including the algae that cause "green water" in ponds.)

Nitrate: A compound of nitrogen and hydrogen which is highly soluble in water and is toxic to fish only at fairly high concentrations. (Nitrates are produced in ponds by nitrifying bacteria (Nitrobacter) converting nitrite.)

Nitrifying bacteria: Bacteria which convert ammonia to nitrite and nitrite to nitrate.

Nitrite: A compound of nitrogen and hydrogen which is highly soluble in water and is toxic to fish at low concentrations. (Nitrite is produced in ponds by nitrifying bacteria (Nitrosomonas) converting ammonia.)

Nitrobacter bacteria: The genus of bacteria which converts nitrite to nitrate.

Nitrosomonas bacteria: The genus of bacteria which converts ammonia to nitrite.

pH: A measure of the acidity or alkalinity of water. pH ranges from 0 to 14, with 7 being neutral.  Below 7, acidity increases exponentially and above 7, alkalinity increases exponentially.

Phosphate: A compound of phosphorus and oxygen which is readily taken up by algae and plants.

Phosphorus: An essential element for animal, plant and algae growth.

Photosynthesis: The making of organic matter by plants, algae and certain other organisms by using sunlight, carbon dioxide, water and inorganic salts (including phosphorus).

Phyla: Plural of the word phylum which is a division of one of the kingdoms into which life is classified.
Polyethylene: A type of plastic.

Pressure filter: A mechanical and/or biological filter through which water flows under pressure.

Protista: One of the kingdoms of living things. It includes algae, amoebas, ciliates, slime moulds, water moulds and a number of other forms of life.

Slime algae: Various forms of algae which cover pond walls, plants and water features. (As their name suggests, the algae are soft and slimy making them difficult to remove by hand.)

String algae: See hair algae.

Submersible: Able to operate underwater.

Ultraviolet clarifier: A container enclosing a globe which emits ultraviolet light. (The ultraviolet light kills algae in any water which flows through the container.)

Vortex filter: A mechanical filter that uses centrifugal force to "throw" suspended particles out of the water.

Water Garden: A garden containing water features and/or ponds.

Water quality: The physical, chemical and biological characteristics of water.

Waterwall: Any wall down which water flows.

Waterwall trough: The water container at the top of the waterwall. (Water is pumped to the trough which distributes it so it flows back to the pond as a falling sheet or by trickling down  the wall.)

Zeolite: A naturally occurring clay which adsorbs ammonia from water.



Frequently asked questions about fish ponds and water gardens. All the usual questions about design, construction and maintenance of garden ponds plus some not so usual answers regarding fish pond phosphorus, nitrite, nitrate and algae control.

If you have a question and you can't see the answer here,  email it to:


Q1. What are usually the factors causing algae growth to get out of control in fish ponds?
A1. In freshwater ponds, sunlight and phosphorus. Algae, like plants, use sunlight for photosynthesis. Most of the ponds I have seen that are permanently in the shade don't have excessive algae growth. With ponds that receive ample sunlight, phosphorus becomes the factor that limits growth.

Q2. How does phosphorus get into a garden pond?
A2. Phosphorus comes into the pond:

  • from the soil and fertilizer of plants that are put into the pond.                                                                                   
  • fromthe plants themselves as parts of them die and decay.
  • from fish food.
  • from fertilizer, soil and organic matter, including leaves, that are washed or blown into the pond.
If you are building a water garden pond, I highly recommend incorporating a leaf skimmer box into the design.  Adding  a skimmer box later will be much more expensive but I would still do it if I had a severe leaf problem.

Q3. Doesn't nitrogen also encourage algae?
A3. Nitrogen, like phosphorus, is essential for algae growth. In freshwater ponds, however, nitrogen usually isn't in short supply because of the activity of certain bacteria called cyanobacteria (so called "blue-green algae"). These algae-like bacteria draw nitrogen from the atmosphere, which is 78% nitrogen. The limiting nutrient for production of algae and plants is therefore usually phosphorus.

Q4. Why do we worry about nitrogen control?
A4. Nitrogen control is important in ponds which have dense fish populations. Free ammonia and nitrite, which are compounds of nitrogen and hydrogen, are highly toxic to fish. Toxic forms of nitrogen are converted to less toxic nitrates by nitrifying bacteria. These bacteria attach themselves to any surface area which is well aerated. If your pond has more than just a few fish, you can greatly increase the numbers of nitrifying bacteria by installing a biological filter. These filters supply a far greater amount of well aerated surface area for the bacteria than the pond provides.

Q5. What is the best material for building garden ponds?
A5. I am not a builder but I can tell you what I have seen in the hundreds of garden ponds I have worked in.
  • Liners are the cheapest way to make a pond, but preformed fibreglass or polyethylene ponds last much longer and are far tougher than liners. Most liners are easily holed by sharp edges.
  • Concrete ponds often eventually develop leaks. If i did build a concrete pond, I would make it 15 centimetres ( 6 inches) thick and reinforce it with galvanized steel mesh.
  • Rendered brick is also prone to crack and leak in time. Many of the paint-on sealants that are applied to the render soon begin to peel away. I don't know whether the fault lies with the sealant or with the person painting it on.  If I could afford it, I would personally prefer to fibreglass the render. Otherwise, make sure the render and paint-on sealants are done properly - you can always install a liner if leaks develop later on.

Q6. What is the minimum size for a pond for fish?
A6. If the pond is in the shade in a cool climate, it can be as small as you like. In a warm climate, if the pond is exposed to the sun for much of the day, it should be at least half a metre deep and at least two square metres in area. The minimum size for a pond depends also on the species and numbers of animals to be kept in it.

Q7. What is the optimum pump size for a garden pond?
A7. According to pond pump manufacturers, the pump should turn over the pond's volume at least once every two hours. I don't know what research they base this on but for now I accept it as a general rule for the average pond. The pump should also be large enough to supply the flow you want for your water features.

The height to which you pump the water above the surface of your pond (i.e.the height of your water features) is just as important as the volume of your pond. Pumps used in ponds are "low head" pumps - they lose pumping capacity rapidly as the height to which they pump increases. When buying a pump, you should ask to see a graph of the pump's pumping capacity against the water's "head" (height above the pond surface).

The pumps I have used never seem to perform as well as the companies claim. That is possibly because of more friction in the pipework and equipment in ponds than in the ideal squeaky-clean laboratory conditions in which the companies do their tests. Usually, I work on double the recommended pump capacity. If the rate is a bit high, don't worry. As everything "gunks up" the flow will be reduced. Don't use a pump that  pumps far too much. You can throttle back the pump or divert part of the flow back into the pond but over the lifetime of the pump you will waste hundreds of dollars in electricity.

Q8. How often should I clean my filter?
A8. As often as possible to remove filtered organic matter before it breaks down and is released back into the water where algae can assimilate it. Realistically, filters should be cleaned at least once a week but I know that most pond owners do it only once a month, if that. The filter will keep on removing suspended matter until it becomes totally clogged. That may take months but suspended organic matter that is filtered out will quickly break down in the filter converting bound phosphorus into a form that can be taken up easily by algae.

Q9. Do pond pumps need to run continuously?
A9. The main reason for running a pump is to aerate the water. If you run the pond continuously, the pond will be healthier and won't develop the unpleasant odours of toxic gases such as methane, ammonia and hydrogen sulphide (rotten egg gas). These noxious substances are produced by anaerobic bacteria (organisms that live only where no oxygen exists).

The most important time to run your pump is actually in the early hours of the morning.  Dissolved oxygen in the water is at its lowest level at this time because algae and plants only produce oxygen during daylight. At night,  all of the living things in the pond including algae, plants and bacteria (exceot for anaerobic bacteria) are still using oxygen causing the dissolved oxygen level to drop further and further as the night draws on. If your pond is heavily stocked with fish, the early hours of the morning are when they will most need your pump to be running to supply them with oxygen.

If the pond has high fish densities, the number of nitrifying bacteria will need to be constantly at an adequate level to convert the ammonia produced by the fish to nitrates. The number of nitrifying bacteria  will remain constant only if conditions remain constant - the supply of oxygen and food (ammonia) coming past the bacteria must be constant. If you turn your pump off, the nitrifying bacteria will die back and will take days to weeks to regenerate. Therefore, if you have a lot of fish, the pump should run continuously.

Q10. What filters do I need?
A10. I recommend a combination mechanical, biological and ultraviolet filter for most ponds in warm climates. If the pond is in the shade all day and has good plant coverage, you may not need the ultraviolet part of the filter. However, adding a separate ultraviolet clarifier later will be more expensive than including it in a combination filter in the first place. If you already have a pond with a mechanical filter and it gets too murky to see the fish, you definitely need a UV filter.

Every pond should have a mechanical filter to remove suspended matter from the water to clarify the water and to take some of the decaying organic matter out of the pond.
  • If you have a more than a few fish, you should also have a biological filter. A biological filter is simply any material which provides surface area to which nitrifying bacteria can attach and which doesn't easily become clogged. Most fishpond filters on the market do both mechanical and biological filtration. The mechanical filtration media (usually sheets of open-celled sponge or similar material) always comes first followed by the biological media which is usually perforated plastic balls. The mechanical filter also acts as a biological filter but becomes less effective as it clogs up.
  • If you have recurring problems with microalgae making your pond look like pea soup, I highly recommend installing an ultraviolet (UV) clarifier. For most ponds, in the long term a UV clarifier will be less expensive than constantly dosing the pond with treatment agents.
  • If you have a mechanical filter, you will be able to use a  "dirty water" pump. These pumps require almost no maintenance. Other pumps need a prefilter to prevent them becoming blocked. The prefilter actually does a lot of the mechanical filtering and so blocks up quickly in most ponds. If your pond is small with only a few fish, the pump prefilter will be adequate as both a mechanical and biological filter.

Q11. Why do fish often become diseased and die in a new, clean pond?
A11. If too many fish are placed in a new pond, they may be poisoned by their own wastes. Fish excrement and urine produce ammonia as they break down. Ammonia is highly toxic to fish. Bacteria convert the ammonia to nitrite, which is also toxic to fish. Other bacteria then convert the nitrite to nitrate which is only mildly toxic to fish. The nitrate is taken up by algae and plants or converted to nitrogen and oxygen by denitrifying bacteria. The bacteria that convert the ammonia, however, can take up to six weeks to produce the population size necessary to handle all the ammonia in the pond.
To prevent fish from poisoning themselves, you should put only a few in a new pond. If you have a lot of fish and nowhere to put them but in the new pond, be sure to add some nitrifying bacteria starter to the pond (available from pond shops). If you fill your pond from a chlorinated water supply, before adding nitrifying bacteria or any other bacteria to a pond, remove any chlorine from the water by adding a chlorine neutralizing agent and by running the pump for at least a few hours (preferably a day or two).

If your pond has a lot of fish and you give it a complete cleanout, you should clean your filter media in pond water and keep them in a bucket of pond water until the pond has been refilled. As you refill the pond, spray the water in to aerate it to evaporate off some of the chlorine, and add chlorine neutralizing agent. After the pond has been up and running for a day or two, add nitrifying bacteria starter.

You should clean filter material always in pond water and whenever topping up a pond, spray the water in and add chlorine neutralizer. 
Ideally, if the top-up water is chlorinated, it should be "aged" for a day or two in an open tank to allow the chlorine to gas off.

Q12. How many fish can you keep in a pond?
A12. That depends on the technology in place (pumps, filters),  the quality of the pond's water, the species of fish, and the climate of the area. As a general rule, with a pond with a pump running continuously and proper filtration, I wouldn't exceed 70 centimetres of fish per square metre of pond in a warm climate. In a cool climate, you can probably keep more than double this number.

Q13. How often should I feed the fish?
A13. Ideally, you should keep only as many fish in a pond as the natural food production in the pond can support. Every time you feed fish, you add phosphorus to the pond and feed the algae. If you feed your fish, you should give them only what they can eat in five minutes not more than twice a day. Overfeeding fish not only feeds the algae, it pollutes the water making it less healthy for the fish. Feeling sorry for the fish and overfeeding them is like buying hamburgers and french fries for overweight kids. Sometimes being cruel is being kind.

Q14. How much of the pond should plants occupy?
A14. That depends on the intensity and duration of the sunlight reaching the pond. In a warm climate, if the pond is most of the time in the sun, shading three quarters of the pond with water plants will greatly reduce algae problems. The plants shade the water and take up phosphorus which is usually the limiting nutrient in freshwater ponds. The plants replace algae as the base of the food chain that feeds the fish. Plants also absorb nitrates, heavy metals and other toxins that would otherwise accumulate in the water and make it unhealthy for fish. The plants will need cutting back occassionally or they will eventually choke the pond leaving no room for the fish to swim.

Q15. What fertilizer should I use for pond plants?
A15.  Ideally, nothing. The plants should be acting as a filter to remove phosphorus and nitrates from the water, so they should be kept "hungry" all the time. If the pond has fish in it, the fish will fertilize the plants. A small amount of fish will fertilize a large area of plants. The only figure I have come across equating plants to fish is one kilogram of fish to fertilize seven square metres of hydroponic plants. It is the mass of the plants that matters, however, not the area. I have seen a single clump of high native bulrushes keep a pond healthy for several years.

If you don't feed your fish, fertilize your plants or have leaves blow into the pond, eventually the pond may actually become deficient in forms of phosphorus that the plants can use. If the plants look like they are in trouble, you can add slow-release, low phosphorus fertilizer directly to the area around the plant roots. Pond shops sell tablets for this purpose. Before you assume that phosphorus is lacking, however, check the hardness of your water. The only thing usually lacking for plants in fish wastes is calcium. If your pond water is low in calcium, try adding some agricultural lime before adding fertilizer.

Q16. Does limestone increase algae problems?
A16. In my experience, usually it does, but not always. In water saturated with limestone, the pH will stabilize at around 8.3 favouring some forms of algae over plants. In ponds starved of phosphorus or any other essential nutrient for algae, the limestone will make no difference. Most ponds, however, have an abundance of essential nutrients. Limestone releases calcium into the water. Where limestone increases the algae, calcium may be the nutrient limiting algae growth in the pond. Another factor could be that limestone releases carbon dioxide as plants and algae take it up. Carbon dioxide is used in photosynthesis. I'm told that injection of carbon dioxide into hydroponic water increases production.

In time, chemical reactions on the surface of the limestone seal it off from the water. When we clean the pond, however, we scrub the limestone clean once again exposing it to the water. If you have limestone in your pond, wash it off with a gentle spray.

Q17. What is the best way to control algae?
A18. I recommend three ways:
  • Shade the pond with anything but trees that drop leaves. Put the pond on the shady side of the house, build a pergola over it, cover it with water plants.
  • Control phosphorus by removing leaves (preferably with a leaf skimmer), not overfeeding fish, frequently cleaning your mechanical filter, vacuuming the sediments from the pond bottom, fill the pond with plants.
  • An ultra-violet filter will totally clear the water of microalgae, the worst problem algae.

Q19. Is water exchange necessary for ponds?
A19. Yes. In time, heavy metals, phosphorus and other undesirable substances accumulate in the pond. The best practice is to vacuum the bottom of the pond with a pond vacuum, regularly (weekly, monthly, quarterly - depending on the pond's needs and the enthusiasm of the owner). Small, cheap vacuums which work off garden hoses are available from pond and swimming pool shops. If the pond is above ground, you can siphon the sediments off with a hose. 

Don't remove more than 25% of the water so when you top up the water quality doesn't change too much and overly upset the established ecosystem. Ideally, if the top-up water is chlorinated, it should be "aged" for a day or two in an open tank to allow the chlorine to gas off. Otherwise, spray the water into the pond to aerate it to remove as much chlorine as possible and add chlorine neutralizer at the same time.

If you don't regularly vacuum the pond bottom, you should clean the pond out completely once or twice a year, or once every two years, depending on the pond.






























News Archives


Copper ionizers for hair algae control: do they work?

Several years ago, copper ionizers hit the market. The principle is sound: copper is toxic to hair algae (also known as blanket weed and string algae) at concentrations as low as three parts per million. Above this concentration copper is toxic to fish so the ionizer has to keep the copper level in the water within a narrow range around three parts per million.

However, copper is toxic only in its ionic form and copper ions readily precipitate out of solution at pHs above 6.8. To work efficiently, the water either has to be kept at a pH below 7 or the water leaving the ionizer has to be rapidly distributed throughout all parts of the fish pond via a number of outlets. Ideally, the pH should be kept down and a network of outlets should be installed in the pond.

All of this is theory. What has happened in practice? Some claim the ionizers don't work (especially in ponds that generally have pHs up near 8). Other complaints include  the lack of battery back-up:  the slightest pause in the supply of power turns them off. Finally, the units are expensive but fragile.

Ideally, copper ionizers should have sensors near the outlets in all parts of the pond with direct feedback to the ionizer. The release of copper ions should be automatically adjusted to each individual outlet to keep the concentration over the whole pond within the required limits. Too sophisticated for the average garden pond? Perhaps, but one day soon copper ionizers may become the solution  to that scourge of the water garden - blanket weed.

Anyone with experience, good or bad, with copper ionizers, please email me at: 

Artemia largely replaced in fish larval feed.

Researchers have succeeded in replacing up to 90% of the live feed in fish larvae diets. Artemia cysts, the only reliable source of live feed for fish hatcheries, will grow in cost as the world demand outstrips supply. Reducing the proportion of artemia necessary in fish larval diets is a major step forward for aquaculture.

Freshwater crayfish growth rate greatly increased.

Selective breeding has doubled the yield of marron (a large Western Australian  crayfish)  potentially increasing profit up to nine-fold.

The information in both the above news items were from the Fisheries R&D News, vol.14, no.1, Feb. 2006.

Whale slaughter

Certain Japanese companies continue to slaughter whales under the farcical premise of "scientific research". Whales are not only in danger of extinction, they are highly intelligent - possibly as intelligent as we are and are certainly as intelligent as a child.  I presume they are therefore extremely sensitive to pain and are as terrified of death as a child is.

One excuse a Japanese spokesman gave for continuing whaling was that much of the world was telling them to stop doing it. It was a matter of pride.  Personally, I believe it is a matter of money.

Governments continue to turn a blind eye to the Japanese whalers. After all, it might cost us money if we upset the Japanese too much.

What can we do to convince the Japanese that they don't really need to slaughter these magnificent animals? The only thing I can see working is to boycott Japanese goods. We need to hurt the pockets of Japanese companies which have more influence on the Japanese government than the whaling companies have.

Boycott sites include:

Global dimming

Recently a TV documentary I saw questioned whether reducing some forms of atmospheric pollution was actually contributing to global warming. Pollutants from hydrocarbon fuels were said to be causing global dimming, reducing the severity of the greenhouse effect. The greenhouse effect is caused by the carbon dioxide from the fossil fuels. Unless the carbon dioxide was reduced along with the other pollutants, global warming may actually be accelerated by removing the other pollutants. In other words, the greenhouse effect is being masked by the other pollutants and is potentially much worse than was thought.

I have been blaming global warming for the increase in nuisance algae in ponds. Now, I'm wondering whether, in the near future, we will have any water for the algae to be a nuisance in.

For more information see:
© LloydMathews, June, 2005

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Copyright © Lloyd Mathews 2005

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