Another way to get any glue residue off 99% of things is use peanutbutter :P
I know it sounds odd, but its a home remedy we use here often - it even took hot tar off my car's bumper!
I normally peel the stickers off, then put some smooth or chunky peanutbutter on the areas that have glue on it. I rub it in for about 2 or 3 mins then let it stand for 10 and then rub again for 2 mins and then wash it off.
It works like a charm and its peanutbutter - nothing bad in there for the fish once its been rinsed off.