About nitrite and nitrate: Nitrate ions (NO3–) are formed from ammonium (NH4+) as the end product of the nitrification process. Nitrite (NO2–), which is highly toxic to freshwater species in particular, is produced in the first stage of the bacterial process. If the nitrification process works, nitrite is converted to the comparatively non-toxic nitrate in the second stage. Increased levels of nitrite above 0.05 mg/l (ppm) are usually found in tanks that are in their maturation phase or in tanks with nitrate filters. Nitrate affects the quality of the water in the aquarium: Higher concentrations of nitrate in saltwater aquariums will end up inhibiting the growth of delicate coral. An algal bloom is often the result of a high nitrate level in an aquarium.
Some reef aquariums have ultra-low nutrient conditions – nitrate shortages can occur in cases such as these. Therefore, the nitrate concentration in the aquarium water should be tested regularly. It is advisable to keep the nitrate concentration in saltwater aquariums below 20 mg/l (ppm). When nurturing hard coral, the aim is to keep the nitrate concentration below 10 mg/l (ppm). Lower limits for nitrate depend on the general conditions in the aquarium.