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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.