Painted black background.
 

I'm just curious why people paint the back of their tanks black? Does it serve a purpose? I've always used the backgrounds you see at the LFS. :-?

I just painted my new 55g I am setting up. My reasoning is those backgrounds from the LFS can let water seep between it and the glass and it's cheaper. Also, I think those backgrounds look fake anyways.

A lot of people, especially with planted tanks, use black paint or construction paper as it emphasizes the plants and gives the tank depth. Me personally I still have a background from my LFS but once my plants grow in I'll probably go the black route

It looks better, it offers a sense of security for the fish because there is only one pane of glass exposed to the outside world, it helps make fish and plants pop against the contrast of the black...