As copper raises, corals will bleach and polyps will stop extending. 10-100ppb (parts per billion, not million) is typical I think, natural copper from calcium and alk, but added copper gets obsorbed into rocks and coral. Making those rocks unusable in a reef tank due to leeching. It can in essence nuke a tank when copper additives are used. Even says so on the packaging iirc. It can also kills off inverts. He has several different systems with separate sumps.