I have used the No.28 medium blocks for years and they were awesome for maintaining calcium...however my Ph ran consistently low...but with no problems. I ran into problems when I decided to ditch the blocks and buffer and mess with Alk and Ph. BTW, I use tap aqua. I'm finally back to coral recovery after returning to the blocks and running GFO.

IMO, these blocks kick butt for adding calcium, and my corals from SPS to softies thrived, but that’s just my experience.