I've used Tropic Marin Bio Calcium as a calcium additive for nearly 5 years and have had great success with it.
When I set up my last tank, I had coralline growing on the glass within 3 months.
The bio calcium is added to make up water through the sump. While I'm no chemist, it does not appear to have any ill effects on corals or alter water quality.
Hope this helps.
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