You could also sprinkle some sand onto the silicone, to give it a grout kinda look. I saw something on a website, I think it was reefzone, where a guy used a two part epoxy, mixed sand with it, and formed a bottom. He used wax paper to break it up into pieces for removal. Maybe something along those lines? How big is the tank? It might be possible to get a piece of slate or granite cut to match your tank bottom. Try places that do countertops or tabletops, but expect to pay. Another option, more labor intensive, but more creative, is to take a piece of foam, carve it to whatever countour you want, coat it with epoxy, paint, and silicone to the bottom of the tank. Works well for backdrops, don't see why it wouldn't work for the bottom.