black background for sure... but you also have to choose the bottom surface treatment and be mindfull of the viewing angle. if you're looking down the bottom will also influence how you see your fish.
I used to have bright backgrounds but now my favorite is black.
I have a very dark "midnight blue" on mine! I agree that dark colours make the fish stand out (unless, like me, you have a black p.poleni that becomes camoflaged at the back! Lol!)
as weird as this sounds i have to agree with it i used to paint my tanks blue then i got one painted black and have done it ever since and it hides pesky algae too LOL
I use light blue on most of my tanks because of my discus and angels. Discus will darken against a black background. But I do use black on my 210 South American tank. The only fish that I find that questionable with are green/notatus severums -- they tend to show their full vertical bars most of the time. When you move them to lighter colored tanks the bars fade away and they look better IMO. Light-colored sand in the tank really seems to help. I've seen people with black backgrounds and black substrate and it really darkens the fish.
depends on the fish you keep. but for most fish and most of the time, black is the best background color. as other said it brings out the fish color with good lighting. and sometimes dark background isnt good and can cause the fish to darken up....fish like discus.