There are many marine aquarists successfully keeping corals nowadays and a good thing it is for protection of the reefs because of commercial interest, a pool of home grown or ‘fragged’ corals and enjoyment.
Those aquarists, particularly with hard corals, will be adding calcium probably along with other supplements. This will often be done in a smaller aquarium by using a commercially prepared additive and with a larger aquarium by employing a calcium reactor.
So it is clear that aquarists know that calcium is an essential and also know that it must be available in sufficient quantity at all times. This is for the benefit of shrimps and snails and the like but also mainly for corals. The aquarist faithfully does a calcium test and uses the additive or replenishes the calcium reactor with media from time to time. Some aquarists could answer the question ‘What is calcium added for?’ with ‘It allows corals to grow’. That’s correct.
A hard coral when dead is usually white. The colour that is seen on a living coral is caused by zooxanthellae, the single celled algae that dwell in the flesh of the coral. It is for these algae that lighting of the correct spectrum and power is provided. The algae provide a high percentage of food to the coral but are also involved in calcification.
As with most other things, looked at in depth it is all quite complicated. It could well be interesting to those with a bent for science but for most of us the basics are fine. There isn’t a need for complications on aquaristsonline.com; the point is to keep it straightforward. Therefore, a link is provided below that gives a basic run-through of corals in the aquarium including calcification:
For those who would like to delve deeper the link below should be of interest: