This is going to sound really stupid, but if you like a black background......... bin liners!!! I have done it to one of my tanks, I would put a piccy up but the camera battery is dead! Basically I wanted to darken my hospital tank so I shoved a bin liner behind it, its got crunched up and looks almost like a 3d rock background. Plus being plastic it wont get watermarked.
I have done similar with one of my big tanks using a cheap tablecloth (one of those thin plastic wipe down ones) it only cost 99p to do a 4ft tank.
I have done similar with one of my big tanks using a cheap tablecloth (one of those thin plastic wipe down ones) it only cost 99p to do a 4ft tank.