I found myself in the same boat....wanting a bare bottom tank, had all the sides and back painted black, but I was gonna use black muslin fabric underneath the tank on the stand (had some laying around). Needless to say, after filling the tank I did not like the looks of it at all, so I covered the bottom with sand. Doing it again, I would have tried painting the bottom first. If I then filled it and did not like it, I then would have probably used granite tiles for the bottom (which I should have, especially since I have about 10 laying around

). But try painting it as opposed to putting it on top of something black was the point of my post.