Turn off all of the lights in the room except the tank lights. Use a tripod, and no flash.
Are those just overturned black flower pots half-buried in the gravel? That is such a cool idea as hiding places that still allow you to see the fish!
I had never thought of puting in a background, but now that I think about it, it would be nice for hiding the filter hoses and camoflaging the black heater... I think I just might try that in my tank.