I've been doing my paper developing in trays in my bathtub. I've gotten some reddish-brown stains on the tub
If the stains are organic from developer then Clorox applied to a sponge and left to sit on the stain may remove it.
For silver residue try silver polish.
If the stains are metalic then try some tub stain remover from the hardware store.
After that things get - interesting:
Google for "chromic acid", "piranah solution", "persulfate sulfuric acid"
"permanganate sulfuric or nitric acid" ... These may leave the stain in place and instead remove the house. They work on glass, if the vitreous in your bathtub is truely virtuous they be OK, if not they will make a _real_ mess.
Chromic acid is the old laboratory glasswear cleaner and used to be recomended by Kodak but pouring chromium down the drain is considered a no-no these days.
I second the recomendation for Bon Ami ... scrubbing with regular cleansers will leave fine scratches in the bathtub and then the stains will >really< stick.