Hey, thanks for the input. I think I'll save the concrete idea for my next project. I have started to silicone earth/coco fiber type substrate to the background. (what a mission!!) I was going to do the same thing with the waterfall but now I like the sound of Toms idea. I have been using aquarium silicone, but at around £10 for a tiny tube it could get a little expensive for my wallet. The only black silicone that I can find is alot better value for money, but says it is not safe for aquarium use.
Are you using a special type of black silicone?