Nitrite is just as toxic as ammonia. It affects the oxygen carrying capability of the blood. Because it also irritates their skin, koi may jump out of the water and flick or rub themselves on the pond bottom or walls. These are symptoms of high nitrite levels and if exposed to them for a prolonged period, koi will become listless and die.

Keep nitrite to an acceptable level by carrying out water changes to dilute it. Continue daily water changes and testing until zero nitrite has been reached. As with high ammonia levels, reduce feeding during this time to cur down the load on developing filter biomass.

This is the end product of the nitrogen cycle. Although it is less toxic than ammonia or nitrite, do not allow the nitrate level to exceed 100 mg/l and aim for a reading of 50 mg/l or less. A separate vegetable filter or a planted pond can help to reduce nitrate levels.

A planted watercourse will take up nitrate and phosphate which is another fertilizer from the water and may help to reduce the amount of filamentous algae present in the pond. Vegetable filters will require maintenance in autumn and spring and have a minimal, if any, effect during the winter months when the plants die back.

If a planted water course is switched off in the colder months to minimize cooling, give it a thorough clean before reintroducing it into the system to prevent polluting the main pond.

The test kit uses chemicals in tablet form to measure the nitrite content of the water. Some test kits use liquid chemicals to produce the color change. After an appropriate time has elapsed, compare the color of the water in the test tube to that on the chart to ascertain the level of nitrite in the water.