Nitrates (NO3)

Nitrates are the end product of the nitrogen cycle. Less toxic than ammonia and nitrites but harmful if they accumulate over time.

Detailed explanation

Nitrates represent the end of the nitrification chain. They are the least toxic nitrogenous compound but not harmless. In nature, nitrates are diluted by huge volumes of water or consumed by plants. In the aquarium they accumulate.

For the reef, low nitrates are essential. SPS corals tolerate 5-10 ppm at most. Above they begin to lose colour, turn brown, the tips become discoloured. Soft corals tolerate more, up to 20-30 ppm without obvious problems.

However, be careful of nitrates that are too low in the reef. Under 1 ppm corals may bleach due to lack of nutrients. The ULNS (Ultra Low Nutrient System) trend of recent years has caused more harm than good in many aquariums. A balance is needed.

In freshwater aquariums the story changes. Aquatic plants consume nitrates as fertiliser. A planted aquarium with good plant growth can keep nitrates close to zero without intervention. Without plants, nitrates rise and regular water changes are needed.

Nitrates tend to accumulate in koi ponds because koi produce a lot of nitrogen. Marsh plants in the plant filter help but are rarely enough on their own.

Reference values by aquarium type

How it is measured

Drop test (Salifert for marine aquariums, API For freshwater aquariums) or strip tests (less precise but convenient for the pond). ICP analysis for maximum accuracy.

What happens when it is too low

Only In marine reef aquariums: corals fading due to starvation. In freshwater aquariums this is not a problem.

What happens when it is too high

Uncontrolled algal growth. Brown corals. Long-term stressed fish. In ponds it favours the blooming of green water (unicellular algae).

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