runaway calcium levels

[wingedfish]

My tank is 6 months old and since january my calcium levels have risen from 460 to 600. Five days ago I tested and found 580 cal and 8 Dkh 7.8 Ph. Added Kent buffer (washing soda) for ph and alk thinking the corals would use up calcium and got it to 8.2 and 12 Dkh a day later. Today tested and fount 8 Ph 10 Dkh and 600 Cal. Some sps is bleaching but everything else looks fine. I don't dose anything besides spirolina food pellets. Mag levels were also high at 1600 but have since dropped to 1450.

How detrimental is this problem? Where is it coming from?

Is there a way to drop calcium levels besides massive water changes? Is there a way to force calcium carbonate precipitation with high alk (proportionately high ph) and not hurt anything?

I'm thinking 20% daily water change for a week should get to purer water. I'm using Kent salt which mixed to 1.026 yeilds 9 Dkh and 480 calcium.