I used muriatic acid to clean a bunch of old dead rocks for my 55g restart. I used a large rubbermaid garbage pail, wore long chemical resistant gloves, mask and goggles. I put all the rock in garbage can, put enough water to cover the rocks and slowly added the muriatic acid. I let the muriatic acid do it's thing for about an hour. I then used a box of baking soda to stop the process. I turned on the hose and rinsed everything for about 30 to 40 minutes. Took the rock out and laid it on the driveway and sprayed some more. I let it dry on my deck for 3 days and put it in my tank to start cycling. I was amazed how clean the rock came out. It was pretty gross and smelled awful before that. I would do it again in a heartbeat. First pic shows the start of the muriatic acid. At the end it was very dark and full of gunk. Next 2 pics are of the rocks all clean and ready for the tank.I was thinking of doing MA but I hear that it is very hazardous