Calcium Reactors

Q) Is it absolutely necessary to have a calcium reactor in my reef aquarium?
John Sullivan, Maldon, Essex. England.

A) It really depends on what type of livestock predominates. Soft corals and anemones for example do not have a calcareous skeleton and have little need for high calcium levels. Consequently, they tend to do very well in aquaria where calcium is in short supply.

On the other hand, hard corals, particularly Acropora spp., and other reef-building corals positively DEMAND high concentrations of calcium to survive and grow (as well as intense lighting, etc.). For these creatures, calcium is best provided in the form of calcium hydroxide, commonly referred to as “kalkwasser” ( translated literally as ‘limewater’).

Calcium reactors are now readily available to dose the aquarium constantly and keep levels stable. Alternatively, two pints of reverse osmosis water mixed with a teaspoon of calcium hydroxide powder can be mixed together in sealed plastic bag and hung over the aquarium. If a corner is pierced several times with a pin, the enriched water will drip into the tank and boost calcium levels very effectively. Of course, a careful check on the specific gravity must be made at the same time.

Calcium levels are best maintained at between 400-500 mg/l and this can be confirmed by regular checks with a quality calcium test kit.

Both fish and coraline encrusting algae benefit from a high calcium content and there is some evidence to support the fact that the introduction of calcium hydroxide helps reduce phosphates as well.


Don’t use calcium additives and pH buffers at the same time. One will often cause the other to precipitate out of solution, causing the water to go milky. In addition, the benefits of both will also be severely reduced. Leave at least 24-48 hours between the two treatments.

© Nick Dakin. May not be reproduced in whole, or part, without permission.