If you are going to be useing the rock in fresh water then pore some bioling water over the worst infected rock this will kill everything on the rock as nothing will live long in the fresh water anyways.
I did buy some rock a few years ago and 1or 2 pieces had Atapsia on it an a couple did come off in the bucket that the rock was in. I dumped the salt water down the drain and put fresh in it and the atapsia lived for a couple day butthen died. They are resilent little buggers.
If you want everything out of the rock then biol for 1 hr and then rinse a couple of times this will kill everything and remove some of the critters. I would not want all the critters in my rock to be rotting in a fresh water setup.
Bill