So, I never thought anything of this, just thought it natural, but, I've noticed that my larger base rock (which I have laying across two rocks to form a tunnel) and One other rock next to it turning charcoal almost black. It's the same rocks I've had a tough time keeping corals alive on. I had lost a couple months back a large Stylaphora and birdsnest, (they both had gotten quite large when they died). Is there something that would cause rocks to turn charcoal gray / black? There are patches of coraline algae, but it's very small patches. . . These are large 20 lb rocks that have turned gray / black (maybe larger).