I actually have an experiment set up on my desk here in the lab ready to go using marco rock and a couple of different extracting solutions (DI water being one of them), I just haven't had time to do the extractions. The only scientific paper I've read on the topic was written from the perspective of using aragonite as a phosphate sink to reduce eutrophication in Florida, but as far as I know, no one has specifically written a paper on ways to reduce the levels of phosphate present in live rock. I a
Randy Holmes-Farley wrote a couple of articles on phosphate for advanced aquarist and reef keeping magazine that I just went back and read (
http://reefkeeping.com/issues/2006-09/rhf/ and
http://www.advancedaquarist.com/2002/9/chemistry) one of the main points is that in solution phosphate both precipitates out as CaPO4 due to the fact that our tanks are usually supersaturated from the perspective of that mineral, and it also adsorbs right to the surface of the aragonite, also at an ideal pH of around 8.4 I think. So long as the aragonite crystals aren't growing (which would be the case if CaCO3 was precipitating out of solution on to the surface of your live rock or on to your heaters), that adsorption is reversible. If the crystal is growing, the phosphate will get incorporated right in to the physical structure of the rock. The adsorption capacity of aragonite, it turns out, is also salinity dependent, which is something that I totally forgot. This is because sea water changes the ratios of the different forms of inorganic orthophosphate, so at identical pH's in fresh vs seawater, there will be very different breakdown of phosphate species, and not all forms of phosphate are equally likely to bind to aragonite.
Randy's article was also written from the perspective on ways to export or immobilize phosphate in an aquarium, and not from the perspective on ways to deplete existing rock of it's phosphate store, but based on one of his recommendations I can extrapolate a couple potential avenues to explore -
1. Randy suggests that keeping a pH of over 8.4 may help to keep phosphate that is bound to the rocks in place. He says that if the pH falls below that, down in to the 7s, or even further to the point where some of the calcium carbonate beigns to dissolve, large quantities of phosphate can be released from the rock. Perhaps you could try dropping the pH of the water your rock is in substantially, then doing several 100% water changes until you stop seeing the phosphate levels climb. It's just a conjecture, but if you did this at a pH that was lower than you intended to keep your tank at, once the rock was immersed in your display tank, it would no longer contribute phosphate but would instead act as a sink for it.
2. I'm no longer certain about my suggestion to soak in RO water as I'm not totally clear on whether the forms of orthophosphate in fresh water are more or less likely to bind to aragonite. however, if freshwater forms are less likely to bind to aragonite, soaking your rocks in fresh water with a pH slightly below 7 could potentially liberate a whole lot of phosphate from your rocks.
In either case you should test the pH of the water your rocks are in right now, if it's below 8.4, that alone might account for why your rocks are acting as a net contributor of phosphate to the water column. If your rocks have lots of phosphate in them, either adsorbed to the surface or trapped inside the crystals themselves, it might not actually be possible to 'deplete' them to the point where they no longer have the potential to release it back in to the water column, so your solution might simply be to maintain your display at a higher pH so that they're less likely to be a problem, which you can do using limewater as your dosing solution.