well, you can do what I did and stack liquor bottles behind it and then light those with rope lights and a black light (Looks wickeD cool!) My tank sits in the divider between my kitchen and my living room so I didn't really want to go with anything that was completely opaque.
Another thing that works good if you don't want to go with the regulare aquarium background or painting is fabric. Just go to your local craft shop and I gaurantee that you can find something that fits the look you want.