Nitrite is harmful to your fish and can affect their health even when small quantities are present. If your fish are lethargic or seem to be gasping for air an excess of nitrite. To see if that is the case this is a cheap and reliable way to find out. Fish waste, plants and uneaten food decompose and affect water quality. The healthy bacteria in your filter should remove ammonia and nitrite but the pond balance is not always even. To use you simply take a sample of your pond water, add the test liquid and observe the colour of the resultant water (a colour chart is provided to guide your observations).