Frequently asked questions on pond pollution

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Matt Clarke answers some of your most frequently asked questions on pond pollution...

How can I tell if my pond is polluted?

Some fish may show changes in their behaviour if the water is polluted. They usually gasp at the surface, but might also hang motionless in the water, lay on the pond floor or jump out.

However, although the fish may provide some clues, you can't tell how polluted the water is unless you test it with the right kits. Get into the habit of testing regularly for ammonia, nitrite, nitrate and pH, and reach for the test kits whenever fish die, look sick or behave strangely.

What should I do if I detect nitrite in my pond?

Nitrite reduces the fishes' ability to remove oxygen from the water. So increase aeration by ensuring that fountains, pumps and waterfalls are left running continuously.

Additional food creates further pollution and fish go off their food when the water is polluted. Cut back on feeding until the water quality improves.

Perform a series of large water changes to dilute the concentration of nitrite in the water. Use the pump to remove 25-50% of the water each time. Top up the pond using a hose, but don't forget to dechlorinate the water to neutralise the chlorine or chloramine present.

Salt reduces the toxicity of nitrite, which means fish can tolerate slightly higher levels without falling ill. The exact amount of salt required depends on the volume and the nitrite level, but a dose of 1-3g per litre is very effective in ponds, especially when fish are also suffering from diseases.

Keep testing the water for signs of nitrite until the levels drop. Be prepared to make further water changes to help prevent the nitrite rising to deadly levels.

Why should I test the pH and temperature if I find ammonia?

Ammonia dissolves readily in water to form two different chemicals: ammonium (NH4+) and free-ammonia (NH3).

Most test kits measure the total ammonia nitrogen, which is a combination of the ammonium and free-ammonia present. Ammonium is much less toxic to fish than free-ammonia.

The actual amount of toxic free-ammonia present depends on the pH and temperature of the pond water. So you need to test the pH and temperature to find out how dangerous conditions are for the fish.

Some test kits include a look-up chart to enable you to determine how much free-ammonia is present, depending on your temperature and pH level.

Any free-ammonia level is cause for concern, as levels of just 0.2-0.5 ppm can be deadly for many fish.

I've written an ammonia toxicity calculator to help you work this out.

How can I get the fish through an ammonia problem?

Ammonia damages the gills and causes the production of extra mucus. This reduces the surface area of the gill filaments, making it more difficult for the fish to breathe normally.

To aid breathing, boost the oxygen levels by adding extra aeration to the pond. Then dilute the concentration of the ammonia by doing a large water change, and topping up with dechlorinated tapwater.

Place an ammonia-removing chemical filter media like zeolite in the filter to help reduce levels. Recharge this every couple of days by soaking it in a strong salt solution.

Alternatively, you could try APIs Ammo-Lock - a liquid treatment designed to neutralise toxic ammonia.

Keep testing the ammonia and nitrite levels daily, changing more water if levels go too high. Fish may become diseased during and after ammonia or nitrite problems so keep a close eye on the fish for signs of infection.

How should I clean my filter?

Pond filters contain useful bacteria that remove pollutants, like ammonia and nitrite, which arise when things break down in the water. The chlorine present in tap water is deadly to these filter bacteria, so filter media should only be washed in old pond water, however tempting it may be to rinse it under the hose.

If you kill the bacteria by washing the media in tap water, the pond will become polluted with ammonia and nitrite and the fish will suffer.

How can I keep my pond cleaner?

There are a number of pond cleaning gadgets on the market. Of these, a pump-operated vacuum cleaner is by far the most effective. Some models are very expensive to purchase, but many retailers offer a rental service, which may be more cost effective for smaller ponds.

There are a number of biological treatments on the market designed to break down sludge.

Should I change the water?

Many pond keepers don't change their water, preferring instead to simply top-up evaporational losses. However, it's undoubtedly better for the quality of your water to change some of the water occasionally.

Nitrate and phosphate can build up over time, encouraging the growth of the nuisance algae which can lead to green water and blanketweed.

Making partial water changes every so often can help dilute the levels, providing your tap water is already low in these pollutants.

Use your pump to remove about 25% of the water every month or so. Then top up the pond using a hose.

Don't forget to add a good dechlorinator capable of neutralising chloramine as well as chlorine.

This article was first published in the October 2002 issue of Practical Fishkeeping.