You can use a pumice stone. Check it out on Utube.
I love using this homemade cleaner for that. It’s amazing how well it works on toilets too https://chemistrycachet.com/the-best-way-to-remove-mold-mildew/
I've heard Alka Seltzer is amazing.
try coca cola! or vinegar!
WD 40 spray, let sit 15 minutes brush and flush
Pour vinegar in the tank. Let sit for 10 - 15 minutes and flush.