I have no problem with prime/amquel use with copper. Used it many times (3x per day for months) with 50-60 fish at a time in quarantine system. There is a lot of folks on reef2reef and other sites that say things they have no experience with. Ammonia will kill a fish faster then anything. Once the damage is done their is usually no correcting it. Sudden fish death especially when there is multiple fish involved is almost always from ammonia in quarantine tanks. It takes a completely cycled, very mature system with lots of filtration to get that much fish weight/respiration through quarantine. The furan2 tip likely tipped the already teetering scales of biological filitration and nitrite/ammonia took the fish.