Hi Deno, I've used bleach to clean my rocks. I use about a cup per bucket of water and leave them there until clean....I can't remember how long it took but I know it wasn't long. Then I stack them in the shower or under a sprinkler and leave it running until I can't smell bleach. I then dry them in the sun for as long as I can and give them a good sniff to make sure there's no bleach smell.
That took a bit long and always made me nervous, although it never caused any problems, so I now have two sets of rock so that I can put the rocks from the tank straight outside to sit in the sun and rotate sun bleached ones back in.