Assuming you are using 2-part additives, your tank is definitely using calcium, but your test kit is not accurate enough to show that. Alkalinity appears to be needing supplementation because there is less available carbonate in the system, and there is much more available Calcium in the system. So generally people can notice a depletion of Alkalinity, but that same depletion of Calcium might fall within the error range of their Calcium test kit. If you want to learn more about this, read this article by Randy:
http://reefkeeping.com/issues/2004-12/rhf/index.php
Your alkalinity solution is:
- Sodium Carbonate (or Sodium Bicarbonate)
- Calcium solution is Calcium Chloride.
When your stony corals (SPS or LPS) form their skeleton, they use the Calcium from #2 and combine it with the Carbonate from #1 to form Calcium Carbonate, which is the hard part of the skeleton. As such, calcium and carbonate are nearly always used in a 1:1 ratio. When I read all the various articles, I took down in my notes that for each 1dkh of Alkalinity drop, I needed to add 19.3ppm Calcium to ensure things balance. This is just what I could assess would be an appropriate balance from the information online, but it could be 20ppm or something like that.
Most Alk and Cal supplements out there are dosed in a 1:1 ratio. Meaning for every mL of Sodium Carbonate you add, you simply also add 1 mL of Calcium Chloride. Note that Sodium Bicarbonate is not dosed 1:1, since you need twice as much Sodium Bicarbonate, it would be dosed 2mL of Bicarb to 1ml of Calcium Chloride to achieve balance. Most additive vendors do the 19.3 (or 20)ppm calcium per mL to correlate to the 1dkh of Alkalinity for you right in their mixture concentrations.
This is particularly difficult to understand for people using Red Sea Alk and Calcium supplements, since this is one of the very few supplementation schemes out there where the Alk and Calcium are not balanced to each other by concentration, so you have to find out the number of dkh and ppm in each mL and then extrapolate what ratio to dose them. Incidentally, by my calculations, Red Sea users would need to dose 1mL of red sea Calcium supplement for every 0.34mL of Red Sea Alkalinity used
So, to summarize, you likely are noticing a need to supplement Alkalinity and not Calcium because your drop in Calcium ppm is too small for your test kit to notice (within the margin of error). It is likely over time that you would notice your Calcium dropping very slowly. If you dose Calcium at 19.3ppm per dkh of Alkalinity you use, you would not see the Calcium drop very slowly over time.
