Bare bottom for now Gina, when I originally envisaged a marine tank I could see white sand but I've been told it doesn't stay white. I was a bit nervous putting the rock straight onto the glass and I still feel it would be safer to have it sitting on some kind of substrate. For now though, there is something below one of the rocks thats gathering the sediment and pulling it into a pile, the glass base makes it really easy to see it and it's fascinating to watch.