Hi Kelly, I had a remodel with an old fiberglass shower that was NEVER cleaned. After hitting it with The Works toilet bowl cleaner I also used a razor blade and that finally took it all off!
Here's what I use:
Fill a spray bottle with a 50/50 solution of Dawn dish soap and vinegar. Spray it on the hard water stains and let it work. If they are tougher stains, you may have to scrub with a plastic scrubbie!
I agree with Ann. Equal parts of vinegar and water. You could also soak a rag in this mixture and wrap it to fit handles and spouts.
Vinegar and a mild scrubber usually gets that right off!