I posted this with Ron Shimek, but since we're on the subject I'd like to for you guys to take a shot. I have several tanks with a common sump, all of them have sand beds. In the 30 gal breeder the top layer of maybe 1/4 inch hardens up. It has a pinkish hue to it. Really the only difference between this tank and the others is lighting. Its the only one with T-5, 2 blue, and two 10k. I think its coralline growing on/in the sand. Never heard of this happening before. Any ideas or experiences with this?