SPS tank, where? Reef Crystals is made by instant Ocean. Instant Ocean has a higher dKH than Reef Crystals and a lower calcium level. It is impossible to have a high DKH and a high Calcium. In order to have a really high calcium level you must lower your dKH and vice versa, to have a high DKH you must have a lower calcium level. Both can not be high, so a intermediate level is chosen for you by Instant Ocean as in for people who want a high calcium level they get it but they get a level so high that it requires the lowering of the normally high dKH to obtain it. I use Instant Ocean which has a higher dKH, but also use Kalkwasser. To obtain a higher Calcium level I add a little bit of white vinegar to the water I use to make my Kalkwasser and add about 25 to 30 percent more calcium hydroxide. This is the only calcium supplementation I use. I maintain calcium levels at 425 to 450. I have a consistently high dKH (typically 12), but not excessively high. You do not divulge your dKH, but as long as it is 10 or more I would not worry. I would consider switching to Instant Ocean though if you use Kalkwasser continous for all make up water as Kalkwasser used long term tends to lower the dKH and Instant Ocean has a higher dKH that Reef Crystals. This would be a much cheaper alternative to a Calcium Reactor, but a reactor does increse a tanks dKH when used long term.