Well, don't be too disappointed. I think the Nitrate reducing ability of live rock is overrated. I believe that water must be trickled though live rock exactly like it is in sand beds. To accomplish this there needs to be critters living in the rocks and pulling water through as they go in and out of their tunnels. The critters can take years to build their tunnels and once they die out I doubt they are easily replaced in a captive environment. So, I kinda believe that live rock is typically really only useful for nitrate removal for maybe 3 - 5 years. It's still very good with other types of filtration though.
My suggestion is to use the agrocrete for the nitrogen cycle filtration and just find another way to remove Nitrate.