I have a pile of old rocks from my systems in a corner of the garden. When I want to change things up, I go out, grab whatever one s look good, rinse them in the utility sink and give them a scrubbing with a toothbrush to remove whatever will come off. Rinse them again and put them in the tank.
Can you bleach (will turn rocks whiter and kill whatever is on them)? Yes. Can you acid wash them (outside, with eye protection, gloves and clothes you don't like anymore? (will remove the top layer of rock if the acid is strong enough) Yes. You can also "cure" them in a tub for a few days/weeks. It all sort of depends on just how much work you want to put into it.