is said to remove heavy metals (copper) very slowly over time. You need to remove/replace the GAC
very frequently to prevent the possibility of it leeching back into the water.
IME: If you plan on having inverts or a reef I'd tear the whole thing down and start over. Your rock has likely absorbed some of the cooper and will release it over time. As with the sand bed and possibly the silicon in sealing the tank.
I'm not sure that copper kills the bacteria colony. If it did, how would one ever treat with it; even in QT?