Instead of using suckers to stick it to the back glass, use magnets. Just glue them to the inside layer of the stitching canvas grid and cover entirely in silicone. Then use magnets on the outside of the tank. This means the wall will be entirely flush against the glass.
O||O
O is a magnet
| is canvas and glass
I'll have some pics up in a couple of days to make it easier to understand.
Oh, and not my idea by the way so I disclaim credit!