Easy fix .....
Use any boat hull acid cleaner, we use Starbrite Ez On Ez Off because it is readily available around our area. It removes the stains from fiberglass without hurting it, same with sinks and tubs. To do whole tubs, I buy a cheap spray bottle and then throw it away when I'm done...
Kobalt 204-Piece Household Tool Set with Hard Case $ 99
Kobalt 204-Piece Household Tool Set with Hard Case
Item # 782577 Model # 64054
Reg $ 199 now $ 99
Too new of a member to post Brickseek link. Apparently not qualified to post a link yet ..... ;)
I'll update later.
Good luck