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I don't think I'd mess with it- is it even approved for use as a sanitizer? I vaguely remember reading that one of the components of Baqua Spa had it as an ingredient, and if that's the case, it's an even better reason to stay away from the stuff. How does your friend know how much to use and how often? How do they measure the level?
To address your concern as to if it is safe. I use it straight as a mouth wash. I put it right on cuts. But as how it reacts to rubber and other seals I don't know.
Baqua tends to have problems with polycarbonate plastics. I have a suspicion it might be the Hydrogen Peroxide they use for shock. It is 7%, BTW.HS does not recommend using Baqua products in their older tubs because the impellors and light lenses were made of polycarbs. The newer tubs are fine.