Nitrate (NO₃) is produced from the oxidation of nitrite by the bacteria Nitrobacter spp. (Lewis & Morris 1986). Nitrate within aquaria is generally dismissed as not being a threat to the health of fish, although many aquatic species are sensitive to high nitrate concentrations. There are indications that toxicity of nitrate is related to its effect on osmoregulation at high ion concentrations.

Recommended levels

Recommended maximum levels vary quite a bit, depending on a large species-specific variation in nitrate tolerance. Shrimp (e.g. Caridina/Neocaridina species) reportedly are quite sensitive to high nitrate concentrations, a maximum level of 20 mg/l NO₃ is often recommended for these species. For most tropical fish a maximum level of 50 mg/l should be maintained.

How to avoid high nitrite levels

Nitrate can be removed from the aquarium system through 3 mechanisms:
(1) uptake by plants. So, as recommended for removal of ammonia and nitrite, use a sufficient amount of plants in your tank.
(2) denitrification.
(3) flushing.
In aquarium systems, nitrate is primarily controlled through flushing and to a lesser extent through plant uptake.

References

Lewis, W.M. Jr., Morris, D.P., 1986. Toxicity of nitrite to fish: a review. Transactions of the American Fisheries Society, 115, 183-195.