I'm actually in the process of this. I used stuff from the pet store as it was all I could find. I actually did the bleach for 24 hours, then sprayed it off with my hose, dechlorinated water for a few hours, and then an acid wash.. I probably should have waited but my readings were saying that the acid would get rid of the bleach... probably toxicly but it is outside.
We are sitting in the acid wash now. I'm probably going to have to take a pressure washer to it next, pending I can coax my husband to get it out and get it started (the one we have is finicky and belongs in the trash). It took me over 2 years to get him to get the rock out of the tank, so I may be pressing my luck.