From a post by jackerspackle.
Fill a bucket with mild, non-chlorinated water and put in the rootball/growrock cluster..
Gently knead the mass underwater until the rootmass separates. pull it out..
Some bits of root may remain, so to get them out put the rocks in a kitchen colander or seive and spray them with water.. the roots tend to collect at the bottom.. Then just turn the colander over and dump out the rocks.. the roots will be in a nice neat pile on top..
Don't worry about getting every last root hair out.. they'll eventually decompose and get strained out by the prefilter which should be on your ebb/flow return and water pump..
It's important to be gentle to the rocks.. They're just like an aquarium "wet/dry" filter - beneficial bacteria will grow on them as long as water flows over them.. The microbes overpopulate and often directly attact diseases like pythium & fusarium.. Another neat trick they do is convert the ammoniacal nitrogen in some ferts to nitrates.. great for those whose use chems like Shultz or Miracle it Grows..
Just don't use peroxide or chlorine, because it will kill them..