Diy
I just figured out a new way to make a cool custom background. Go to tap plastics and buy a sheet of the plastic that is kind of frosted looking, get the one that is stiff enough to stand on edge but thin enought to cut with an exacto knife. Don't cut it to size til after you paint it so you can choose the section you want by holding it behind the tank and moving it around.
Get a few spray cans of different color of paint (like a dark and light blue for example) and work in a ventilated area or wear a paint mask. Shake the cans and make sure paint is not dripping by spraying on a piece of cardboard first. Then spray the paint on super thinly at a low angle so it is hitting the frosted texture of the plastic from the side. Try for a fade effect. When scuba diving the water is lighter near the surface and darker below, and it gets bluer. Conversely, the sky is darker overhead and lighter near the horizon. Both can look good. On a long tank a dark to light fade across the distance could enhance, or a lighter area could ehance a focal point.
I didn't wind up keeping my background on my ADA tank because of yellow paint drips and because the orange I used looked funny above the waterline, but on a covered tank it would have looked great. I can stick it back on and take a pic if people are interested. I might do another one when I get fish.
My original motivation was to go from black on the left to a sunset orange/white/purple gray on the right. The black side would hide my stealth heater. If you do not go for full coverage you can still light it from behind.