I would say that the rock would leach po4 back into the tank. not sure what will help it other than getting new rock or boiling the old rocks witch i am sure you don't want to do. i would personaly say just get run the reactor with gfo and keep doing water changes it will come down sooner or later.
I am in total agreement with rambler! Give it time! I am actually in the same boat as you due to my neglecting my reef for over 6 months.
Be patient, and continue to run that gfo in your reactor as well as the water changes. You will see it works.
When I first started my first s/w tank 11+years ago, I used tap water. I had no access to the internet, and knew nothing of these wonderful forums so my advicers were Petland & Fishtown.lol When I found RC I read about ro/di water, but I didn't believe that it would be all that different so I continued to use tap water until about 4 years ago.:bigeyes2:
When I started the 155 I did fill the tank with ro/di water, but used the same rocks. The P04 was through the roof. Two years later it was in the neighborhood of 1.50+. Kept up with ro/di w/c, and incorporated a
TLF reactor with phosban, and started bringing them down. I did mess up like I said above so obviously they went back up.
Be patient my friend we shall succeed, and beat the P04 nightmare! And with that we will also beat the cyano, and what ever else is going on in our reefs.

My P04 are down to .23 checked with the
Hanna Checker, and already my my sps are coloring up nicely.
