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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.

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. 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.

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