What Is Nitrate?
Nitrate is the waste by-product of nitrifying bacteria (nitrobacters), which develops in the third and final phase of the nitrogen cycling process. It is what makes an aquarium's biological filtration system function and stay in balance.
Why Is Nitrate a Problem Element?
When nitrate is allowed to accumulate or build-up to high levels it can effect the health of the animals you are keeping, and because marine plants and algae feed on nitrate, this is one of the main reasons problems with algae blooms occur.
What Is an Acceptable Level?
The optimal amount of nitrate in any type of saltwater system is an immeasurable one, but an acceptable range for fish-only tanks is from 10 to 40 ppm. Although fish-only tanks may run at much higher levels, sometimes with no ill effects, this is not recommended. In reef systems even a minor level of nitrate can cause damage as well as death to delicate corals, anemones and other invertebrates, as well as some crustaceans. The acceptable range of nitrate for reef tanks is 0.25 ppm, but not more than 5 ppm.
Other Sources of Nitrate
Even though nitrate is a natural element in aquariums, when doing water changes and topping-off the tank to replace water lost from evaporation, if using unpurified tap water, and/or a brand of sea salt mix that may contain a high level of this element in it, instead of reducing the nitrate, you can just be putting it right back into the aquarium. Therefore, it is wise to filter tap water before using it, and choose what sea salt you are going to use carefully.