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Ammonia in an aquarium is caused by the decomposition of fish waste, decaying food, and dead fish that have not been promptly removed. Ammonia can cause a great deal of problems if not taken care of. It is the No. 1 killer of tropical fish. It is easy to treat if water conditions are ideal. You need to do routine water changes, siphon the gravel , and be certain to keep from overfeeding. With an ammonia level of 6 ppm, the death rate may be as high as 50%. As the levels rise, the death rate will be higher. Ammonia effects fish by causing the blood to loose its ability to carry oxygen. The creates stress and lowers the fish's resistance to bacterial infections such as fin and tail rot, body slime, eye cloudiness, body sores, and more.
It is best to do routine water changes and monitor the water conditons with a test kit made specificially for testing ammonia. More information can be found on water conditions here.
Below is a helpful guide to the appearance of the fish and the action that should be taken:
Testing the water routinely for ammonia is a good idea. There are different test kits available to accomplish this task!