Sorry I didn't chime in sooner; I just noticed your post! Hope this reply isn't too late to be any help.
What did you mean by "poofing"? Don't know what you're talking about there, unless you mean it "poofs" out of existence.
The scum line (or waterline ring) is caused by accumulated organics. That's stuff like sweat, body oils, etc. A scumbug or scumball is a special sponge that helps soak up the stuff that causes the scum. I have a "Zorbo" which I bought from Spa Depot. Same principle as the sponge types; Spa Depot also has scumbugs and scumballs.
www.spadepot.com
An enzyme treatment is really helpful in preventing waterline ring. It may be an extravagant luxury for most pools, but it's almost a necessity for a hot tub. The enzymes aid the sanitizer in clearing out the scum (enzymes "digest" the organics). There are a number of brands; Spa Perfect by Natural Chemistry is one. I like Scumbuster which I get from Spa Depot (no, I don't work for them or get a kickback!!). Scumbuster is MUCH cheaper than Spa Perfect and works great. There are other brands too.
In case you're tempted to use Pool Perfect or some other brand of pool enzyme treatment in your spa, I've been told that the spa products are especially formulated to handle a higher level of organic contaminants.
As far as cleaning your existing scum line. . . there are some products that can be safely used, and yes, there is one that can be found at Spa Depot! Theirs is called Clean-All. However, I have not tried this product, I just know of its existence and the claim that it won't adversely affect your spa's pH or whatever.
I think baking soda works too.
There are other products and sources out there, but these are the ones I know about. I've found the Zorbo and Scumbuster to be very helpful and have never had a waterline ring in my spa.
