You can always use a color wheel, The basic colors of your fish, plants, etc, and use there contrasting color on the back drop. This brings out the best in each color. And with paint you can do a "Fade" like the sunset colors, yellows descending to oranges, to red. Blue to purple to black. This would bring out the best in you fish as they move against the different levels of color, thus not limiting yourself to the many different options. You have have 3 or 4 all in one.
