Iodine (I)

Iodine is essential for the moulting of crustaceans and for the health of some soft corals. It is consumed quickly and must be replenished in aquariums with many invertebrates.

Detailed explanation

Iodine in sea water is present in different forms: iodide, iodate, molecular iodine. The total concentration in nature is approximately 0.06 ppm.

Shrimps, crabs, hermit crabs need iodine for moulting. Without sufficient iodine, moulting fails and the crustacean becomes trapped in the old exoskeleton. In aquariums with many ornamentals, iodine consumption is significant.

Some soft corals (Xenia, Anthelia) seem to grow better with iodine supplementation. Sea fans also benefit.

The problem: Iodine is nearly impossible to measure accurately using home tests. Iodine drip kits are notoriously unreliable. ICP analysis does not always measure it accurately because it is volatile.

For this reason, supplementation is typically done at constant low doses rather than by correction. Products such as Lugol Solution (iodine/potassium iodide) are dosed in drops. An excess of iodine is toxic, so it is better to dose little and regularly.

Regular water changes with quality salt provide enough iodine for most aquariums.

Reference values by aquarium type

How it is measured

ICP analysis (with precision reservations). Drop tests (Salifert) exist but are considered unreliable by the community.

What happens when it is too low

Failed molts in crustaceans. Soft coral receding. Reduced growth in sea fans.

What happens when it is too high

Toxic for fish and invertebrates. Never overdose. Signs of excess include corals suddenly closing after dosing.

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