I had photos printed in large format to fit the back of my tank. They were taken in (part of) their natural habitat. It looks good but it wasn't cheap necessarily. You could do similar by drawing or painting a scene. It doesn't have to be a photo background at all. I have it taped to the outside of the glass and it has held up pretty well.
I made my own background out of foam and grout ended up costing over $100 and I don't really like it anymore and kinda wish I did it a bit differently.
The one I would do now which would cost around $50-$100.
it's called a spray foam background. All it really is, is just spraying the foam on your tanks walls, covering it with silicone adding things to it like cork bark then covering it with dirt, all and all you get a nice realistic background which I would of loved to done but sadly can't =[ (Plus my tank is wooden so I couldn't really do it anyways)
heres a video that will explain it much better, this is just a way easier one then I did and cheaper. https://www.youtube.com/watch?v=svEB0eTe5Cg&t=75s (Also once you apply the foam wait till it's completely cured then wait another 3 days in case if fumes are still lingering before you do anything else)