That's one of my plans... The aquarium has cross-braces, so to hide them and brace the actual tower structure at the bottom I'll be using some marine ply that will be the same rough dimensions as the braces, with some faux rocks made from grout, to make some of the platforms. Also going to make some of those fake vines like in the DIY thread, one of them I'll entwine a low-wattage heat cord I have sitting around, to make a sort of heated night time warm-ish spot to rest on.
@Aztec Wow I forgot about this thread. I'll take some pics tomorrow to show how the ledges are fixed.
For light fake rocks I'd use polystyrene with a thin layer of tile grout over it, hit with a paint brush or similar as it was drying to create some nice texture.
Still haven't actually finished this thing. So close to completion but so much to do.