How do you remove lime stains from a fiberglass bath tub
CLR = Calcium, Lime, Rust remover
What Walt said. Or try magic erasers
Lime-Away. Last time I used it on a fiber-glass tub it worked fine; no damage to surface. That was a while ago; in case the formula of the cleaner has changes, you might clean just a small out of the way spot to check.
try soft soap...
Boat dealerships have great fiberglass combination cleaner/polishers available that work.