Calcium is a very important element in the water in a reef aquarium. It is required (along with carbonates/bicarbonates) by calcifying organisms which include but are not limited to:
Normal seawater has a calcium concentration of around 400 mg/L. Calcium levels in an aquarium with calcifying organisms should be maintained at levels from around 350 to 500 mg/L.
Calcium can be maintained by a number of methods
Calculating calcium usage
There are a few of easy ways to get an approximate idea of how much calcium is being used by the organisms in a tank.
The more accurate method is take two measurements of the calcium concentration exactly 24 hours apart with no addition of any calcium between the two readings. If you normally dose with Kalkwasser overnight, a morning reading would be appropriate and you should miss the Kalkwasser dose for that night. If you use a calcium carbonate reactor, turn it off for the duration of the measurements. The difference in the calcium concentration of the two measurements will give you the approximate usage of calcium by calcifying organisms in the aquarium. This figure can be used to determine how much calcium needs to be dosed.
Another method would be to take one measurement just before the lights come on in the morning and one just after lights out at night, and before you start dosing with Kalkwasser. This method assumes that the bulk of calcification occurs when the lights are on. This is true for symbiotic stony corals and calcareous algae, but there may be other calcifying organisms, such as snails calcifying at night.
If you know the rate at which calcium is being added to the tank in any 24 hour period, you can determine the calcium usage rate simply by taking two readings at exactly 24 hours apart. Add the calcium addition rate to the difference between the two readings and you will have the calcium usage rate.
Further Information
For more information on the chemistry of calcium and its relationship with alkalinity, see the following articles:
Last updated: June 1, 2003