If it's an Acerbis tank off a Husky or KTM then you are better off searching for another tank that is not as badly stained.
I have tried scraping with a razor blade, bleaching with both chlorine and hydrogen peroxide. Sometimes it helps, sometimes the staining is right through the plastic and can't be removed. I have several tanks hanging in the garage that could not be restored due to bad staining.
Painting is an option ....as a last resort.
I never leave fuel in my white plastic tanks, siphon it out after every race meet.
Good luck