Salifert Nitrate Test Kit
When an aquarium has insufficient regions deprived of oxygen (= anaerobic zones) or denitrification proceeds incompletely then nitrate will build-up.
A too high nitrate concentration can result in unwanted algae-growth and these might irritate corals in their neighborhood.
Nitrate as such also retards coral growth.
The nitrate concentration in a proper functioning reef aquarium should preferably be lower than 1 mg/L. Fish-only aquariums would usually show much higher nitrate concentrations.
Most test kits use a procedure in which a certain part of nitrate is converted chemically into nitrite and is measured as such.
Once again, just as with nitrite testing, amine interference has a major impact on the measured value. If such an interference occurs concentrations much lower than actual will be measured.