Nitrate is a primary food source for algae.
Nitrate levels above 10 ppm often cause and fuel rapid algae growth in the aquarium. Green water (a free-floating algae bloom) is also a very common result of elevated nitrate levels in the fish tank. Once algae are present, persistently high nitrate levels will cause the algae to overtake and become oppressive. This is what’s known as an algae bloom. At this stage nitrate is often overlooked as the cause because levels may be low when testing the water. A low nitrate reading during a severe algae outbreak is due to the algae taking up nitrate at an equal or faster rate then it is being produced.
Nitrate affects the health of aquatic life in the fish tank.
Some aquatic life has very low nitrate tolerances. More sensitive species include invertebrates, corals, and fry. These can be adversely affected from short-term exposure. Almost all aquarium livestock, including fish, experience serious consequences when exposed to high nitrate levels long term. The effects of nitrates on fish are twofold. The direct effect is physiological. Nitrates lower antibody production, affect the composition of the blood, cause kidney damage, decrease fertility, and also affect osmoregulation (ability to regulate fluids). A more indirect effect is stress. Nitrates, and the resulting decline in water quality cause stress on fish. Stress reduces the slime coat making the fish more vulnerable toward disease. High nitrates also contribute to lower dissolved oxygen levels in the water.
Rising, or persistently high nitrate levels indicate and cause problems with water quality and consequently fish health. Nitrate levels should be kept at the lowest level possible and ideally should never exceed 10 ppm long term.