Sorry, I missed your earlier reply about the size of the tank.
You said the sponge filter is there in case the powerhead fails. That should be the other way around. You should have a filter running to take care of the bacteria to kill the ammonia and nitrites.
Did you cycle this tank? How long have you had it?
As Mikey said, you should never boil rocks, but pouring boiling water on them or gravel is a good way to disinfect, so long as the rocks/gravel are not in the tank at the time you do it.
Have you added everything back in the tank now? How are things looking? Fish acting normally?