"best" hot tub cleaner?

I'm pursuing cleaning a spa tub that keeps spitting out black and white greasy- soapy chunks (I call 'em old-people chunks, based on the folks I bought the house from), and a local spa shop recc'ed some 'super concentrated chlorine', a teaspoon in 110 gallons supposedly kicks grime's butt. It is sold primarily as a sanitizer. I joked (1/2 anyway) about dumping half the thing in my tub and they said the fumes would probably kill me.

This stuff (its granular) carries a warning about getting it damp- it'll catch fire if it gets moist. That's how you can tell its the good stuff, I guess.

Haven't tried it yet, or I'd give you an evaluation.

Dave

Teenage boys with the promise of the keys for the car this weekend.