Another product to add to Tropic Marin’s Bio Actif system brand is the pro coral kalk. Quite simply itÂ is calcium hydroxide in its purest form and is used to make up the classic limewater. Tropic Marin Pro-Coral Kalk increases pH and calcium content to promote the growth of corals and calcareous coralline algae. It inhibits the growth of undesirable algae by binding carbonic acid, one of the inevitable by-products of operating a COÂ²- calcium reactor. Over in Europe where this is now available you can get a tube of the stuff for $19, hopefully that price will drop if/when it gets imported over here to the US.