Cleaning goldfish bowl...

I have a single goldfish that I keep in a approimate 1 gallon bowl. (I know
it is to small, but that is the best I can do for right now?) I change the
water every 3 or 4 days. Today after I emptied the bowl there was like a
scum around where the water line was. I tried to remove it the best I could
with a clean paper towel, and re-filled the bowl. Is there a way to get this
cleaner? I know that I am not supposed to use cleaners.


Any help greatly appreciated
Bob