Buy a black background from your LFS? Coat one side with vege oil and stick it to the back of your tank (making sure not to get any in the tank). Then squeegy out the air bubbles. I did that with both my tanks, i find it gives it a really nice look.
Oceanvisions self adhesive background. no two ways about it. I've tried the oil thing the petroleum jelly thing, the paint thing. just buy this stuff stick it on and away you go. best looking background I've ever had. very little work very inexpensive. Search a thread in this forum called "finally a background that really works"
dave
this is cheap but i thought i'd throw it out there: for my 10 gallon, i just got 3 pieces of black construction paper and taped it back there. sounds cheesy i know but it actually looks really nice and didn't cost me a cent. probably only works for small tanks like 10-20 gallons though.