Calcium Demand in the Tank Question

[Sea Witch]

I've read a *lot* about calcium demand and how to compensate when your calcium levels, your alkalinity, your magnesium, and your pH are all falling.

My question is what can you have in a reef tank that doesn't have such a high calcium demand so you don't have to keep replenishing beyond what your instant seawater gives (isn't it about 500-550ppm Calcium?) gives you?

Are there some kinds of reef tanks that don't need additional calcium beyond the reef water salt?