I freshwater cleaned them for about a day pretty good, but I guess I probably didn’t do a long enough. To be honest, I’m not sure if I rinse the substrate much because it was super super super fine. They were in containers close sealed containers but that might be it. What do you think I should do? Lol.I once used years dead white Coral skeletons as a substrate for coralline algae. I assumed they would be pretty much blank, but in reality they had left over organic material that decayed and pushed my ammonia much higher than I was targeting in my small containers, maxing out my ammonia test.
So yes, I totally would believe that years dead rock from the garage could have plenty of organic material to generate those ammonia levels.