or, if you have either an old-school slr or a new fangled digi with remote, you can just leave the camera infront of the tank and remote release the shutter
I've got a longish remote release for my slr that I mostly use with telescope, maybe I'll try it with the fish sometime, to get a more 'natural' look rather than the 'FEED ME ... FEEED MEEEE!" display ... seriously, they beg worse than dogs!
ps
an older epson digi I used to have access to didn't have a remote, but it did have a 'remote control' via usb option, which is cool because you could zoom in/out, focus, control flash, adjust f/stop and other things, including operating the shutter, guess you'd need a notebook or other computer pretty close to the tank to use that method though.