I have a chlorine spa, stabilized with 50-70ppm CYA and usually running around 3-6ppm Chlorine (bleach) with 50ppm Borates (in case I let it go, which I do sometimes -- no alge yet). I'd go bromine, but it kept poofing when we left the cover off. I keep the spa around 90-93 when not in use to discourage the bugs. I occasionally treat the spa with a chemical that is supposed to cause the bugs to sink. I noticed a significant scum line develop around the water line. What causes the scum line? I read a few suggestions about getting a scum ball. Where can I get one and which is the best to buy? Also, what is the best way to clean the scum line off without damaging the acrylic?