Dry rock was once wet, and full of organic life. When you add this to your system, everything that is on the surfaces of the rock will once again be in contact with water. This will leach into your water column in the form of phosphate, PO4. Usually when we talk about nutrients in the reef, we're talking about Nitrates. But depending on the types of corals you plan on keeping, PO4 will stunt their growth as well.
To get this out of the rock, first you'll want to hose it off & remove as much of the dust as you can. Using a standard garden hose will do the job. Then you'll want to soak it in circulating RO freshwater for a few days, drain it, & repeat the process 3 - 4 times. Circulating the water will allow the PO4 to come off the rock surface & reach equilibrium in the RO water. If you have the ability to test for PO4, you should see this number falling after each bath. Don't use salt as you are just throwing it away.
Our system is using about 400lbs of rock & 380 of it came dry. We skipped the step to remove PO4 from the base rock & it eventually grew out using a DIY algae turf scrubber & clumps of green hair algae from the display tank. We could have sped up the process but we don't do water changes. It would have been much easier to remove it from the beginning.
Enjoy the build!
