How do you remove dark stains from a fibreglass shower base?

Dampen the pan and coat it with a layer of borax or ZUD. Scrub after about 15 minutes and rinse completely.

I just dealt with this myself last week.
I tried making a paste from Comet and other claeners to no avail. While in the hardware store, I saw a product made by CLR used to clean plastic lawn furniture. Worked great!