Assuming you've done this by now, but if you're trying to reseed the rock, you would want to do that in salt water. If you're trying to clean it, well, there are a number of ways to do that.

I scrubbed the rock and had 4 powerheads on it for 5 days. I changed the water everyday. I have it in SW now and I'm going to add a couple of pnds of rock from my established tank.