Recommended Ranges
Before getting into problems and solutions, let’s first define what constitutes a problem and what does not. Based on published studies3 involving the calcification of corals and other organisms, I recommend the following:
Alkalinity2 (due to bicarbonate and carbonate but not borate, so those using Seachem salt must raise this value substantially to accommodate the borate being counted in standard alkalinity tests)
2.5 - 4 meq/L or 7 - 11 dKH or 125 - 200
ppm CaCO3 equivalents
Calcium:
380 – 450
ppm calcium ion or 950 - 1125
ppm CaCO3 equivalents
Proud sponsor of this column
If you are anywhere within these ranges for both parameters, you do not need to perform any correction on your tank chemistry, though you may choose to do so for other reasons. In this sense it makes no difference what the relationship is between the two values. If alkalinity is 4 meq/L, it is not inherently any “better” for calcium to be at 380
ppm or 450
ppm. Also, these ranges are somewhat arbitrary, especially at the high end. In fact, the primary reason for having a high end at all is that it is often difficult to keep one of these parameters above the minimum end of the range if the other is over the top end. So if one of these parameters is slightly above the high end, and the other is OK, that is not a problem worth worrying about.
One of the reasons that you may find compelling to adjust values even when within the recommended range (or outside but close to it) relates to test kit errors. All measurements of calcium and alkalinity have some uncertainty associated with them. Even if the kit is a reliable one, you may still want to strive to be in the center of the range to make it less likely that you are actually outside of it and only appear to be inside of it due to uncertainties in the measurement. This issue is especially important at the low end of the ranges, and not so important at the high ends.
