I got my rock from Eli, not sure if OP is using live rock or dry rock... but here's what I did.
Removed as much dry dead matter as I could reach with tweezers.
Bathe in 1 part bleach to 10 parts tap water for 24 hrs with a powerhead.
Bathe in 1 part Muriatic acid (pool section at Cdn Tire) to 10 parts tap water for 4 hours, or until chemical reaction (bubbles) stops.
Sprinkled lots of Arm & Hammer baking soda (to neutralize acid), I let that sit for 1 hour. Stirred a few times.
Hosed down rocks and left them out to dry, few hours.
They all came out sparkling white with just minor spots of brown.
They did sit dry, in a box, inside for about 3 months before I was ready to aquascape.
Began cycle, tested PO4 at 0.03 for a month.
After a month PO4 dropped to 0.
Don't want to jinx myself, but I think there won't be any problem with leeching PO4 in the future. If there is I am ready for it.
I credit my success (so far) with the bleach/acid baths and the use of the dry rock. I'm not sure how much long term PO4 values would be if it were previously live rock.
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