What are your thoughts on a "frosted" look on the back part of the tank? I know there's some sort of film that you can apply to the outer part of a tank that creates a "frosted glass" effect, which makes the tank relatively clear, but potentially distorts out the life support equipment...
The frosted thing is something I have not considered, that is an interesting idea.
I guess it also depends on how you can search for aquarium backgrounds. There are some cool 3d aquarium backgrounds, which assist in showing depth, but can restrict what you're going for to an extent. You could also try for something like a moss wall, which acts as a background, but has plants in it....what are your goals? I assume all the "oceany" backgrounds are out, but do you want something that reflects an image of a planted riverbed or something?
In my minds eye it's hard to see anything with an actual image, whether drawn, painted, or computer generated, to fit the bill, even it if it is a picture of "underwater river grass" or something relevant. I'm not sure something like that could ever look real enough that it would not distract. Perhaps if it had images of the same plants as in the tank, and displayed in a manner that creates an illusion of a seamless continuum off into the distance. That would be some sort of custom or DIY project.
For an off-the-shelf product, i'm thinking something subtle like a gentle gradient of a single color, that get's darker with depth, to create the illusion of depth without stealing any focus from the tank. Maybe more sophistication would be to add some slight hint of a god ray through the water as if from a point source to further add to the illusion of natural light (e.g. sun/moon). Or maybe something subtle that creates an illusion of water movement. Many possibilities.
If you really want to get fancy you can us a lighted background. Not sure if it's sold in your tank size.
If I have a whitish wall beyond my tank I usually go without a background. Black looks good depending on setup to me.
That is one cool product! The larger one would fit I think. At $430 its about as much as the acrylic tank it would go on, rather pricey. Going to check some youtubes on it though. I think its outside my price range but an interesting find none-the-less.