Sulfates

Ions naturally present in sea and fresh water. In marine aquariums they are the third most concentrated component after chloride and sodium.

Detailed explanation

Sulfates (SO4) are present in natural seawater at approximately 2700 mg/l. They are the third most abundant anion after chloride and bicarbonate. Their direct biological role is limited but they contribute to the overall ionic balance of the water.

In most marine aquariums sulfates do not require monitoring or supplementation. Synthetic salts contain them in adequate quantities and water changes keep the level stable.

An excess of sulphates may indicate the use of sulphate-based supplements (magnesium sulphate, potassium sulphate) in excessive quantities. In this case the solution is to switch to supplements in chloride form.

In freshwater aquariums sulfates may be present in tap water in varying concentrations. They usually don't cause any problems. Magnesium sulfate (Epsom salt) is sometimes used to raise GH without raising KH.

Under anaerobic conditions sulfate-reducing bacteria convert sulfates into hydrogen sulfide (H2S, smell of rotten eggs). This occurs in very deep, poorly managed substrates or dead zones without circulation. Hydrogen sulfide is extremely toxic.

Reference values by aquarium type

How it is measured

Only via ICP-OES analysis. There are no drop tests for sulfates in aquariums.

What happens when it is too low

Rarely a problem. It may indicate a poor quality synthetic salt.

What happens when it is too high

Per se non-toxic. It may indicate excess sulphate supplements.

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