Phosphates (PO4)

Phosphates are nutrients that in excess fuel algae growth. On the reef they must be kept very low. In planted freshwater aquariums they are needed for the plants.

Detailed explanation

Phosphates enter the aquarium mainly from food. Uneaten food, feces, organic debris: everything releases phosphates. Some cheap synthetic rocks release them for months.

In the reef, phosphates inhibit coral calcification. A coral in water with high phosphates grows more slowly and loses its bright colours. Coralline algae stop expanding. On the other hand, algae love phosphates and proliferate.

There is an optimal ratio between nitrates and phosphates in the reef: approximately 100:1 (the so-called modified Redfield ratio). If phosphates drop to zero while nitrates remain high, or vice versa, cyanobacteria or dinoflagellates are favoured. The balance between the two matters more than the absolute value of each.

In planted freshwater aquariums, phosphates are a macronutrient. Plants actively absorb them. A Dutch aquarium with heavy plant growth may require phosphate supplementation.

In ponds, phosphates favour green water (algal bloom). Marsh plants help absorb them but in the warm months production often exceeds consumption.

Reference values by aquarium type

How it is measured

Hanna Checker drop test (the most precise for marine aquariums, measures up to 0.01 ppm). For freshwater aquariums the JBL or Sera kits are sufficient.

What happens when it is too low

Only in marine aquariums: faded corals, thin tissue, zero growth. Dinoflagellates may take over because they compete better than algae in ultralow nutrient conditions.

What happens when it is too high

Algae everywhere: glass, rocks, substrate. Corals losing colour. Green water in the ponds.

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