I like aquascape backgrounds *that don't show any fish on them*... just plants and rocks. Dark blue or blue gradient backgrounds would be my second choice, and that's what I want to get for my SW tank... it isn't a reef, so all the SW backgrounds with coral pics would look stupid. I have blue gravel or a mix with blue in it in all my tanks except my SW and the 20g I just set up... but for a natural looking tank, I'd prefer dark brown gravel. The 20g has play sand, which looks great.