i soaked my old fw rocks in a 10% bleach 90% water solution for a few hours, then i flushed them with fw for an hour and soaked them for the rest of the day. then i flushed them again the next morning, and dried them for another day. they are doing great, in fact they have picked up more coraline than most of the others. JUST DONT OVER DO IT WITH THE BLEACH. a little goes a long way.