Bleach

What would be a general bleach to water ratio for getting the stains out of my plastic fermentor? 1 Tablespoon per gallon?

More. Like half a cup. Rinse, rinse, rinse, and rinse.

Assuming a 5 or 6 gallon bucket the proper use of bleach would be to fill your bucket up to the top (5 gallons) and put in 1/4 to 1/2 a cup of bleach, then mix in 1/4 to 1/2 a cup of vinegar. Allow to sit for 2 minutes and it should kill everything on the surface, allow to sit for 1 hour and it will remove stains. Bleach is a much more effective killer in an acidic environment, hence adding the vinegar, just don't mix the vinegar and bleach directly, that could poison you. Make sure you put them both in the water.

This method comes highly recommended by the inventor of star-san, I heard him talk about it on a podcast, but I can't remember where it is right now.