That is what I was thinking...my tank had bottomed out phos. It had been 1-2ppm before I started my task to bring it down, rock is 20 years old, never thought I would see the phos disappear.not sure if this matters I used all cycled rocks and media so all live. Maybe my rock already has all the po4 it can handle and yours still absorbs some? I dont know seems like a lot has changed in the 12-13 years I was gone but chemistry shouldnt have lol.