I put the flash on top of the tank, actually two flashes with diffusers, arrange the flashes so the light bounce inside the fish tank and leave no shadow area.
I normally set everything manual, cover the windows and make a dark room, only leave enough nature/tank light to see clear enough through the viewfinder. The actual source of light depends solely on the flashes. The shutter speed normally set long enough to cover the actual exposure which is 1/250 according to the flashes, then I pick an ISO, take several shots at different narrow aperture, settle on the best for full tank shot.
If my subject is the fish, I choose bigger aperture and use a Tamron 180mm macro(PITA, slow and dull focus in the dark).
I never use back drop, the background is naturally dark because the flashes only cover inside the tank.
can you show some of your nature/wildlife photos for our enjoyment?
I have the same 7D II, my primary lenses are canon 10-22mm efs, 17-85mm efs, sigma 50-500mm II, and a tamron 180mm macro.