Iron (Fe)

Iron is an essential micronutrient for aquatic plants. The deficiency causes pale leaves with green veins. In marine aquariums it is less critical.

Detailed explanation

Iron is essential for the synthesis of chlorophyll in aquatic plants. Without sufficient iron, plants are unable to produce green pigment and new leaves grow yellowish or white with the veins remaining green (ferric chlorosis).

In planted freshwater aquariums, iron must be supplemented regularly because it oxidizes rapidly in water and becomes unavailable to the plants. Fertilisers use chelated iron (Fe-DTPA, Fe-EDDHA) which remains available for longer.

The dosage depends on the density of plants and light. A Dutch aquarium with strong light and CO2 consumes more iron than a low-tech one with easy plants.

Red plants in particular require more iron than green ones. The red colouration depends on the production of anthocyanins, a process that requires iron and intense light. To have Rotala rotundifolia very red you need enough iron.

In marine aquariums, iron at low concentrations feeds the algae coralline algae and the refugium. But be careful: excess iron in the reef also feeds unwanted algae. Minimum and controlled dosages.

In ponds, iron comes naturally from the soil and from mains water. Supplementation is rarely needed unless water lilies are grown in pots with depleted clay soil.

Reference values by aquarium type

How it is measured

JBL Fe drop test for freshwater aquariums. In marine aquariums only ICP analysis. Warning: iron oxidizes quickly so the sample must be analysed immediately.

What happens when it is too low

Plants with new yellow leaves (chlorosis). Red plants turning green. Slow growth.

What happens when it is too high

Freshwater: Rarely a problem. Marine: stimulated algae, possible stress for corals.

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