I can't understand how you can not raise calcium I f you are using cal hypo.
I don't understand it either. I used a good drop test kit. Maybe the calcium was low to start with. I should probably test it more often, and plan to do that next year. For now it is getting cold and I won't be using the pool or adding chlorine.

Doesn't calcium naturally go down over time?

I added the cal hypo by broadcasting it on the surface. I have a sand filter.

My big question is about what happens when it sinks to the bottom and sits there for a while before dissolving. Does it do any harm to a plaster pool?

I