Hey i dont pretend to know anything about chemistry, but there is more than one way to make clacium, whether its calcium bicarbonate, calcium hydroxide, calcium chloride, etc. But if you have diatoms in the tank, which would be wierd, they might hold the key to your undoubtedly odd case. Also, you might have a system with organisms that form a chain, processing the calcium until it can be reused. i.e. Coral eating fish -> diatoms coral/calcareous algae -> coral eating fish. (im sure there would be something in the middle but im not THAT knowledgable, although i have been searching..