Ammonia (NH3) is a chemical that is highly toxic to fish even in small amounts. Ammonia toxicity causes severe stress to fish making them more susceptible to disease and death. This chemical also causes damage to gill membranes and inhibits normal respiratory function. It is important to test the aquarium regularly for this chemical; once a month if your tank is healthy but more often if there are signs of sickness or death. When using a testing kit ammonia traces should be undetectable in your results. The test results are measured in ppm or parts per million and your results should ideally read 0ppm.