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Nitrates are the result of the bacterial breakdown of ammonia which turns into nitrite which in its final stage will be nitrates. Nitrates are less toxic than ammonia and nitrites but can cause stress, which can lead to disease as well as death if not properly cared for. High nitrate accumulations are more toxic when oxygen is low. Nitrates can do damage to the fish's red blood cells and this in turn causes them to be oxygen deprived.
Symptoms of high nitrate levels are: new small fish die off within 2 to 3 weeks, fish appear lethargic other than at feeding times, growth is slowed, increased susceptibility to disease, slow wound healing, redness or red patches on the body.
The above chart was taken from a brochure that came in my pH test kit made by Aquarium Pharmaceuticals, Inc.