Two issues occurring here: in terms of equal numbers of atoms (molarity), equal supplementation of calcium and alkalinity is not occurring, AND the utilization of equal amounts is not occurring, possibly due to the buffer action of the bicarbonate/carbonate buffer system to neutralize either excessive amounts of organic acids and/or excessive CO2 in the water system. Our test kits test for alkalinity due to bicarbonate and carbonate include borate (which the regular SeaChem salt uses a substantial portion of to boost its buffer level, o.k. for FO, not so much for reef systems), so those using SeaChem salt must raise this value substantially to accommodate the borate portion being measured in standard alkalinity tests) , so think of standard test values for alkalinity in systems measuring water columns comprised of SeaChem as being 10% high as a rule of thumb, or, if you use SeaChem, get the SeaChem borate test to determine EXACTLY what % of your alkalinity is borate (or use IO, etc.)
Before going off the deep end with the chemistry of hermatypic elements, let's define what might be considered problematic for these elements. For alkalinity NOT INCLUDING BORATE, decent sea levels in NSW should reflect a range of 2.5 mEq/l to 4 mEq/l, or in terms of dKH/l, 7 dKH/l to 11 dKH/l (or 125PPM to 200 ppm CaCO3 equivalents if you use these terms); and for calcium ion (CA++), 380PPM to 450PPM as calcium ion (or 950PPM to 1125 PPM CaCO3 equivalents). If your system reflects levels within these ranges, there is no need to perform any corrective action on your part, system chemistry for the hermatypics is adequate and within a reasonable range for stony coral spp. to perform skeletalization and calcification. In this sense it makes no difference what the relationship is between the two values. So long as alkalinity is somewhere within the range as listed, it is not any better for calcium to be at the lower or higher end of the range, so long as precipitation or excessive abiotic calcification in/on hardware is not a problem. Keep in mind that for the most part, these ranges are only an approximation of what we might see under normal circumstances in a NSW water column, especially as the ranges max out at the upper end. For our purposes, the main reason we have maximums for closed systems 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 (see the archive for supersaturation of hermatypic elements). So if one of these parameters is slightly above the high end, and the other is WNL, then we really do not have an issue.
When using additive directions, especially those by the additive manufacturer, remember that manufacturer recommendations are based on maintaining a tank, not in making substantial corrections. To make such corrections, you may need to add much more than is recommended on the label, so don't take these instructions (on the label) as Gospel. If you are adding a balanced additive like kalkwasser, and the pH is not getting above the top end of the normal range of about 7.9 to 8.5, then the worst that is likely to happen from overdosing is wasting some money and causing some abiotic precipitation of calcium carbonate on the plumbing and heaters (unless you REALLY OD the tank and the pH goes through the roof, but that is an issue for some other post).
On the other hand, if you are using two-part calcium and alkalinity additives, you must be very careful to not create an imbalance by adding too much of one relative to the other (or conversely appropriate amounts of one part while underdosing the other). Directions for deciding how much of these types of additives are demonstrated in
Randy Holmes article at Advanced Aquarist , but you should rely on frequent testing with quality seawater test kits (especially the titration method kits) more than what might be flat recommended amounts by an additive's manufacturer to determine how much of these hermatypic supplements should be added to the tank's water column.
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Originally Posted by Randy Holmes-Farley
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.
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This does not EVEN consider testing errors in methodology by a hobbyist or out-of-date test kits or tests of poor reliability/quality. If the test value is way out of line and you have been supplementing and there is no apparent issue with the creatures in your system, then consider test kit errors before performing any drastic changes to the system. Remember: treat the patient, not the symptoms.
Sometimes you may find yourself adding large amount of calcium and alkalinity supplements, but just cannot maintain the desired values, in spite of adhering to the recommendations of one of the reef calculators. If this occurs, especially over period of days or weeks, then it may be time to measure the magnesium level of your water column. Magnesium plays an important role in preventing the abiotic precipitation of calcium carbonate as you approach levels of supersaturation of calcium with respect to alkalinity. If your Mg++ is substantially down from 1300 PPM, you may be experiencing excessive amounts of calcium and alkalinity loss to this route. Low levels of Magnesium get blamed far more frequently for calcium/alkalinity difficulties than it actually deserves, and usually is the result of excessive or long-term use of two-part additives as leftover sodium and chloride from the separate calcium chloride and sodium bicarbonate/carbonate accumulate and change salinity in tanks so treated. Keeping this in mind, realize that if the Mg++ is off, then most likely, so are the other conservative elements that we don't have test kits for (K+, SO4--, borate, etc.). This is most easily corrected via frequent 10% water changes, and you can use a Mg++ kit to determine when it may be necessary to do a larger water change in order to reestablish the correct proportionality of the conservative elements based on you results from the Mg++ test kit.
I am starting to ramble, HTH.
