<u>Recommendation Details:<span style="color: #ff0000;"> Critical Parameters</span></u>
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</em>. 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 http://www.advancedaquarist.com/issues/mar2002/chem.htm"><span style="color: #0000ff;">natural levels of calcium</span></a> 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.<span style="font-size: 1-1px;">3</span> Exactly why this happens was detailed in a previous article on the [IMG]http://www.advancedaquarist.com/issues/apr2002/chem.htm"><span style="color: #0000ff;">molecular mechanisms of calcification in corals</span></a>.
For these reasons,[B] I suggest that aquarists maintain a calcium level between about 380 and 450 ppm.[/B] I also suggest using a [IMG]http://www.advancedaquarist.com/issues/feb2003/chem.htm"><span style="color: #810081;">balanced calcium and alkalinity additive system</span></a> 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, [IMG]http://www.advancedaquarist.com/issues/nov2002/chem.htm"><span style="color: #0000ff;">adding calcium chloride</span></a> is a good method for raising calcium.
[B]Alkalinity[/B]
Like calcium, many corals also use "alkalinity" to form their skeletons, which are composed primarily of calcium carbonate. It is generally believed that [IMG]http://www.advancedaquarist.com/issues/apr2002/chem.htm"><span style="color: #0000ff;">corals take up bicarbonate</span></a>, convert it into carbonate, and then use that carbonate to form calcium carbonate skeletons. That conversion process is shown as:
HCO<span style="font-size: 1-1px;">3</span>- <span style="font-family: Wingdings;">à</span> CO<span style="font-size: 1-1px;">3</span>-- + H<span style="font-size: 1-1px;">+</span>
Bicarbonate <span style="font-family: Wingdings;">à</span> Carbonate + acid
To ensure that corals have an adequate supply of bicarbonate for calcification, aquarists could very well just measure bicarbonate directly. Designing a test kit for bicarbonate, however, is somewhat more complicated than for alkalinity. Consequently, the use of alkalinity as a surrogate measure for bicarbonate is deeply entrenched in the reef aquarium hobby.
So, [IMG]http://www.advancedaquarist.com/issues/feb2002/chemistry.htm"><span style="color: #0000ff;">what is alkalinity</span></a>? Alkalinity in a marine aquarium is simply a measure of the amount of acid (H<span style="font-size: 1-1px;">+</span>) required to reduce the pH to about 4.5, where all bicarbonate is converted into carbonic acid as follows:
HCO<span style="font-size: 1-1px;">3</span>- + H<span style="font-size: 1-1px;">+</span> <span style="font-family: Wingdings;">à</span> H<span style="font-family: Arial, Helvetica, sans-serif;">2</span>CO<span style="font-size: 1-1px;">3</span>
In normal seawater or marine aquarium water, the bicarbonate greatly dominates all [IMG]http://www.advancedaquarist.com/issues/feb2002/chemistry.htm"><span style="color: #0000ff;">other ions that contribute to alkalinity</span></a>, so knowing the amount of H<span style="font-size: 1-1px;">+</span> needed to reduce the pH to 4.5 is akin to knowing how much bicarbonate is present. Aquarists have therefore found it convenient to use alkalinity as a surrogate measure for bicarbonate.
One important caveat to this surrogate measure is that some artificial seawater mixes, such as Seachem salt, contain [IMG]http://www.advancedaquarist.com/issues/dec2002/chem.htm"><span style="color: #0000ff;">elevated concentrations of borate</span></a>. While borate is natural at low levels, and does contribute to [IMG]http://www.advancedaquarist.com/issues/dec2002/chem.htm"><span style="color: #0000ff;">pH stability</span></a>, too much interferes with the normal relationship between bicarbonate and alkalinity, and aquaria using those mixes must take this difference into account when [IMG]http://www.advancedaquarist.com/issues/dec2002/chem.htm"><span style="color: #0000ff;">determining the appropriate alkalinity level</span></a>.
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 [I]Porites porites</em>.<span style="font-size: 1-1px;">4</span> 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.<span style="font-size: 1-1px;">5</span> 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, [B]alkalinity maintenance is a critical aspect of coral reef aquarium husbandry[/B]. 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. [B]I suggest that aquarists maintain alkalinity between about 2.5 and 4 meq/L (7-11 dKH, 125-200 ppm CaCO<span style="font-size: 1-1px;">3</span> equivalents)[/B], although higher levels are acceptable as long as they do not depress the calcium level.
Alkalinity levels above those in natural seawater increase the [IMG]http://www.advancedaquarist.com/issues/mar2002/chem.htm"><span style="color: #0000ff;">abiotic (nonbiological) precipitation of calcium carbonate</span></a> 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 [IMG]http://www.advancedaquarist.com/issues/feb2003/chem.htm"><span style="color: #810081;">balanced calcium and alkalinity additive system</span></a> 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 [IMG]http://www.advancedaquarist.com/issues/nov2002/chem.htm"><span style="color: #0000ff;">rapid alkalinity corrections</span></a>, aquarists can simply use baking soda or washing soda to good effect.
