Aragonite reef grade sand. It's a mixture of different grain sizes with tiny shells mixed in. Caribsea makes it. About 2.5-3 inches deep is all. Lots of worms and critters live in it, so I am pretty confident it is stirred and aerated well. There is also some LR ruble that I crushed and mixed in with the sand bed. I also put some of the rubble around the base of the reef. I like the natural look it gives. I'd post a picture if I could figure out how to compress it enoght to do so.