I bought a used aquarium along with all the goodies (skimmer, RO, etc.) The tank had housed some dead rock and when the previous owner tore it down he placed the dead rock on a outdoors concrete pad where it got rained on for a year. The rock is quarried coral rock and has no sign of calcareous growths, green algae, etc. - it is snow white. I assumed this rock would be clean but since I was going to use some of it to support Florida live rock, I decided to soak it in water. After a few days, on a whim, I checked the ammonia level - and it surpassed the limit of my device (>0.5 ppm ammonia, using a Hach DR890 colorimeter and the salicylate method.) I'm now in the 2nd week of soaking this rock and ammonia is still high. So - don't assume the dead rock is clean - cure it like you would live rock!