manganese and iron stains on fiberglass float tank

[floatmaster]

In a 8ft dia float tank made of fiberglass we get brown stains above the waterline I think they are iron and especially manganese oxides mixed or transported there with body oils from the floaters. Will ascorbic acid work on manganese or maybe oxalic acid? Or maybe some other reducing agent? Acid cleaning (vinegar) has been ineffective but it removes body fats.

float tank construction fiberglass
solution 90%saturated epsom salt, releative density 1.25
disinfection by hydrogen peroxide dosing after every session
pH 7.2
TA 80ppm
CH 0
CYA 0
FC 0
TC 0
note waterline rises siginficantly with two floaters in the pool.
below waterline, no problem.

 

[frustratedpoolmom]

You can try a vitamin C tablet on the stains to see if it lifts, if it does then you can follow the Absorbic Acid Treatment instructions.