Black backgrounds do tend to make plants and fish pop, but if you're going for a more subtle or natural look I'd go with either a brownish color that would be a similar shade to either the substrate or the table it's on. You can always hit up the paint store and grab some of those little color cards and tape em to the back of the tank to see what you like best.
One of the more creative backs that I've seen had an artificial horizon painted with a darker color at the bottom and a lighter color at the top. The horizontal line was slightly below the tops of the plants and really made the tank look bigger. If you wanted to go this route I'd try to match up the substrate color as close as possible and then pick a blue like the forum background.