Test as many colors as you have access to.
Hang paper, garbage bags, towels.... anything...
Just to test what colors you like.
One of my favorites was a fabric with a print on it. Actually, 2 patterns. Both were black, one has deep green and medium green fern fronds. The other is deep green and medium green marbled pattern.
Just looking at these patterns, they are not very garish, and by the time they are seen through the water with plants etc in front they are really nice.
Just enough color so it looks like a lake or river that keeps on going, but a dark enough background that the fish color up really well.
So, to paint something like that you might have to go to a faux finish sort of concept where you use 3 colors. In my case, 2 shades of green and black.
I have seen some really nice effects with blue backgrounds, too.
Plain black works fine, but can give it that 'cave' effect if the light is not good and bright.