Yes I'll have to keep an eye on my PH I do have crushed coral in my filter because my PH had dropped significantly and suddenly to a 6.6 and the corals have kept it to a consistent 7.2. If I see it rise too high then ill just take the coral out. Its one giant I guess conch shell. The fish seem to love it. they hang out in there constantly now and it looks very nice in there.Bluestreakfl said:They may not only effect PH slightly, but they will also slowly raise GH over time as they dissolve. As long as the tanks not loaded with shells though you shouldnt see a huge difference, and with regular water changes I don't foresee any problems. If you have inverts like shrimp or snails, it will actually benefit them having some calcium being leeched into the water as they need it for their shells.