I just read the following (cut & paste):
Is this true? I just installed a fiberglass pool and have been using (for the past month) Muriatic Acid. If I can't use that, what should I use?Muriatic acid is heavier than water and may etch the plaster in a shallow pool. To eliminate this possibility, pre-dilute the acid first by adding the acid into a large plastic bucket of water, and then pour the acid solution into the deep end of the pool, away from skimmer and fixtures. Muriatic acid is NOT recommended for vinyl-liner, painted or fiberglass pools.