ok....what is this thing about copper having been in the tank?

Copper can be toxic to most corals and invertebrates. Even though the tank doesn't contain copper itself, odds are its lodged in the silicone somewhere and can slowly leach out over time if the tank is reused as a "display" type tank. Since copper (in small controlled doses) doesn't affect fish the tank can be used as a quarantine tank or a fish only tank.

You could cut out all the silicone from the tank and resilicone it. But its still risky IMO.