So I decided to acid wash the old formerly live, then dried dead rock. The latest reading was still over 100 ppm nitrate and 0.5 ppm phosphate after two 100% water changes (no change noted). I made some strong acid!
I managed not to burn myself though, so that's a bonus.
I donned goggles, beautiful yellow rubber gloves, and shorts, tank, and sandals (ha!). I bought two 900 mL bottles of muriatic acid and poured them into 1 1/2 gallons of tap water. I dipped the rocks one at a time, and overdid the first one in about 12 minutes - that's some powerful mix! The rest of the rocks got about a 5-8 minutes dip, tap water rinse, tap water plus baking soda rinse (1 1/2 containers baking soda and about 3 gallons of water), then back into the rock tub filled with RO. A couple hours later I test the RO at pH 7.8 so I guess the rinse was sufficient. I will test nitrate and phosphate in a few days and see if there was any progress.
Here's the first rock getting half dissolved (lol):
