I am a window cleaner and unless the glass is tempered, which fish tanks generally are not, a razor blade will not scratch it. Go to a janitor supply store and pick up a 4 or 6" razor ($10-15). Just wet the glass with a little soapy water for lubrication and go to town. You can press as hard as you can and it will not scratch the glass. Just be careful to keep the soap out of the tank.