Reef Aquarium Water Parameters by Randy Holmes-Farley

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Reef Aquarium Water Parameters by Randy Holmes-Farley

http://www.reefkeeping.com/issues/2004-05/rhf/

Aquarists often ask what water parameter levels make for a successful reef aquarium. This article gathers these recommendations in one place, showing them in tables, as well as the corresponding levels in natural seawater.

Many of the recommendations are my own opinions, and other aquarists may recommend slightly different levels. To make clear the basis for each recommendation, a brief description of each particular parameter's importance follows the tables, along with links to other online articles that go into much greater depth on each subject (click on any blue text for the linked article).

Table 1 shows important water parameters for reef aquarists to control for various reasons. Table 2 shows less important parameters, or those too complicated to carefully control, but about which many aquarists have concerns or questions.

Calcium

Many corals use calcium to form their skeletons, which are composed primarily of calcium carbonate. The corals get most of the calcium for this process from the water surrounding them. Consequently, calcium often becomes depleted in aquaria housing rapidly growing corals, calcareous red algae, Tridacnids and Halimeda. As the calcium level drops below 360 ppm, it becomes progressively more difficult for the corals to collect enough calcium, thus stunting their growth.

Maintaining the calcium level is one of the most important aspects of coral reef aquarium husbandry. Most reef aquarists try to maintain approximately natural levels of calcium in their aquaria (~420 ppm). It does not appear that boosting the calcium concentration above natural levels enhances calcification (i.e., skeletal growth) in most corals. Experiments on Stylophora pistillata, for example, show that low calcium levels limit calcification, but that levels above about 360 ppm do not increase calcification.3 Exactly why this happens was detailed in a previous article on the molecular mechanisms of calcification in corals.

For these reasons, I suggest that aquarists maintain a calcium level between about 380 and 450 ppm. I also suggest using a balanced calcium and alkalinity additive system for routine maintenance. The most popular of these balanced methods include limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the two-part additive systems.

If calcium is depleted and needs to be raised significantly, however, such a balanced additive is not a good choice since it will raise alkalinity too much. In that case, adding calcium chloride is a good method for raising calcium.

Alkalinity

Like calcium, many corals also use "alkalinity" to form their skeletons, which are composed primarily of calcium carbonate. It is generally believed that corals take up bicarbonate, convert it into carbonate, and then use that carbonate to form calcium carbonate skeletons.

Unlike the calcium concentration, it is widely believed that certain organisms calcify more quickly at alkalinity levels higher than those in normal seawater. This result has also been demonstrated in the scientific literature, which has shown that adding bicarbonate to seawater increases the rate of calcification in Porites porites.4 In this case, doubling the bicarbonate concentration resulted in a doubling of the calcification rate. Uptake of bicarbonate can apparently become rate limiting in many corals.5 This may be partly due to the fact that both photosynthesis and calcification are competing for bicarbonate, and that the external bicarbonate concentration is not large to begin with (relative to, for example, the calcium concentration).

For these reasons, alkalinity maintenance is a critical aspect of coral reef aquarium husbandry. In the absence of supplementation, alkalinity will rapidly drop as corals use up much of what is present in seawater. Most reef aquarists try to maintain alkalinity at levels at or slightly above those of normal seawater, although exactly what levels different aquarists target depend a bit on the goals of their aquaria. Those wanting the most rapid skeletal growth, for example, often push alkalinity to higher levels. I suggest that aquarists maintain alkalinity between about 2.5 and 4 meq/L (7-11 dKH, 125-200 ppm CaCO3 equivalents), although higher levels are acceptable as long as they do not depress the calcium level.

Alkalinity levels above those in natural seawater increase the abiotic (nonbiological) precipitation of calcium carbonate on objects such as heaters and pump impellers. This precipitation not only wastes calcium and alkalinity that aquarists are carefully adding, but it also increases equipment maintenance requirements. When elevated alkalinity is driving this precipitation, it can also depress the calcium level. A raised alkalinity level can therefore create undesirable consequences.

I suggest that aquarists use a balanced calcium and alkalinity additive system of some sort for routine maintenance. The most popular of these balanced methods include limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the two-part additive systems.

For rapid alkalinity corrections, aquarists can simply use baking soda or washing soda to good effect.