I was given a tip awhile back. On top of using GFO in a BRS reactor I also blast my rock with a power head once every two weeks. I put a clean filter sock in, hold the power head in my hand and just blast all the rock for a few minutes. Wait about 5-10 minutes to let the water clear up some and then do it again. Then I immediately do a water change and vacuum off the rock as much as possible.
It's amazing how much stuff coats the rock after two weeks. Hopefully that will help me in the long run keep po4 from leeching from the rock.