Nitrites, commonly called nitrates, are a group of chemicals found in koi ponds that are made by a colony of beneficial bacteria that consumes or breaks down the koi waste. Nitrite levels increase during the spring when the pond water temperature increases, or any time a number of new fish are added to the pond. To protect your koi and to prevent causing liver, spleen, nervous system and kidney damage due to nitrite poisoning, you will have to lower the nitrite levels of your pond.
Test the level of nitrites in your pond with the nitrite test kit by placing water into the kit and adding the reactor chemicals, which will change the colour of the water depending on the level of nitrites it contains. Read the results of the test. The test kit packaging will have a colour chart that will tell you what the results mean. If the nitrite level is higher than 0.25 parts per million (ppm), it is too high; you will need to lower the nitrite level of the water.
Increase the water aeration or the water pump setting to maximum setting. If the nitrite level is 1 ppm or higher, add another supplemental air pump to get more oxygen into the water and filter out the accumulated waste.
Stop feeding your koi if you have an established pond, or halve the amount you are feeding them if you are just starting up a new pond.
Stop using any ozone generators, UV sterilizers and foam fractionators or protein skimmers; the associated chemicals will increase the nitrite levels of the pond water.
Change out the water. If you have a nitrite level that is less than 1 ppm, remove 10 per cent of the pond water. Add 0.454kg. of salt per 100 gallons of fresh water, mix it well to dissolve the salt and add it to the pond water. If you have a nitrite level that is between 1 ppm and 2 ppm, remove 25 per cent of the water and add 0.907 Kilogram of salt per 100 gallons of fresh water that you put into the pond. If you have a nitrite level that is higher than 2 ppm, remove 50 per cent of the water, and add 1.36 Kilogram salt per 100 gallons of fresh water that you put into the pond.
Wait 24 hours. Test the water nitrite levels again. If the level is above 0.25 ppm, remove the water and add fresh water with salt as before.
A visual sign that your pond has a high nitrite level is when the gills of your koi become slightly rolled or curled outwards at the edges and do not close flat against the fish's body as they normally would. Another sign is a sudden algae bloom on the surface of the water.
If your nitrite level is 4 ppm or higher, move your koi to a transfer tank and remove all of the water. Clean out the pond and start the pond water again. Do not return your koi to the pond until the nitrite level is below 0.25 ppm.