Purple Up raises dissolved Calcium levels in your tank. Along with dosing some other things. Some swear by it, others will tell you its junk.
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Only going off of what "experts" from reef talks at conferences have stated. Purple up was stated to be basically calcium carbonate , bound up calcium not useable to tank inhalants. Basically supper fine sand. It raises the testable calcium as the acid in the test dissolves it. Again only your test can use it not your tank.
Also from "experts" coralline is one of the main consumers of magnesium in your tank. Thus removing possible available calcium ions for corals.
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