nitrate poisoning

although nitrate is less poisonous to fish than ammonia and nitrites (see nitrogen cycle), in high concentrations it is harmful and may also indicate the presence of other toxic waste products (most aquarium test kits measure ammonia, nitrites and nitrates for the nitrogen cycle, not a wide range of toxic wastes). A level of 50 mg/l is the maximum for fish tolerant of nitrates, ideally the aquarium should have levels below 25 mg/l, and for nitrate-intolerant species much lower. Nitrate poisoning is usually a chronic effect, causing poor health and growth, a reluctance to breed, susceptibility to disease from other causes and is probably the cause of black chin disease, q.v. Excessive algae growth is a sign of too much nitrate. Changing the water is a simple solution and nitrates can be removed from tap water by reverse osmosis or the appropriate ion-exchange resin