Ammonia in an aquarium can come from several sources including fish waste, uneaten food and other decaying matter. Ammonia is very harmful to fish. It will damage fish gills and prevent fish from carrying on normal breathing. Low levels of ammonia will stress fish; high levels will cause fish death. The API Ammonia Aquarium Test Strips contains 25-test strips, easy to read instructions, and information to correct unsafe water conditions. For fresh and saltwater aquariums.