Dont soak in freshwater. Either you dip in freshwater or pour hot boiling water over it should you feel that there is pest inside which refuse to come out even after a quick freshwater dip.
Reason: If you soak in freshwater your whole toliet and home will sink !!!
If you intend to kill the rocks just pour hot boiling water and let it cool down, this way your home or room will not be smelly.
I only did it once as i wanted to kill a mantis inside which it refuse to come out no matter what i did and killed 5 of 8 fishes i had.