dont put boiling water into the fish tank as it can crack the glass.
if you want to boil some water in a pot and then turn the burner off and dump the rocks into the pot, that is fine. you can clean the inside of the tank with hot water, just not scalding.
once the fish are healthy and you are able to remove the rocks, you can soak them in bleach with tap water too but it would be best to run them through a strainer in the bathtub before and after if possible.
this is just my opinion, I know rinsing rocks is alot of work.