Report Boric Acid + Sodium Carbonate Buffer Operation & Intro in Hot Tub Water Chemistry Posted April 18 Sorry for the late reply, yes, it's been 3 months! I kept meaning to check back but I would only remember when I was drifting off to sleep at night and then I would forget the next day. I had never heard the Saturation Index was not applicable to acrylic spas! I knew there was no plaster to worry about but I just assumed balanced water was good for other reasons! Still, I would assume you mean a negative Saturation Index is fine but that a positive Saturation Index could be problematic for metal corrosion? Is that correct?