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.
Many nitrate test kits are prone to amine interference. This can give results much lower than actual.