Need some advice on how to properly clean some rock

[Madreefer]

Just boil them and put them in some RO water. Than put one chunk a week in your sump until they are all in. I put 100lbs of rock in my sump that way a couple of years ago with no issues at all.You already stated that the die off was from something else other than a copper treatment. Your buddy should know if he added copper or not.