purple stains

[zacademy]

Our fiberglass pool has purple stains on it...it happened once before and it was recommended that we not use the 6 in 1 shock system. We are only using chlorine, ph - up and shock as needed per water tests. It started to go away after continuous scrubbing and very frequent vacuuming, but has now begun to appear again. Any idea what is causing this and how to remedy the problem?

[muss08]

Have your water tested for manganese. This mineral produces a pink to purple stain.