How can I clean a badly stained fiberglass bathtub?

What caused it to stain???

HI Virginia - I found this site with a few different ways. Hopefully it helps: https://www.wikihow.com/Clean-Fiberglass-Bathroom-Surfaces

Here's an article that might help you - https://www.hunker.com/13423865/how-to-clean-a-stained-fiberglass-tub

I would try comet and a scrubby it cleans well and removes soap scum and doesn’t scratch

Easy Off No Fume Oven Cleaner in the BLUE can. Works on enamel, porcelain, and fiberglass.