Here in New York copper is kept in the stores tanks all the time. If it were not for copper, there would be no salt water fish hobby. The livestock stores get are crowded and not in good shape due to the stresses of shipping. They are also mixed with fish from all over the place in varying degrees of health and were not fed due to bacteria in the shipping water needs to be kept low. . If you are at the store when they get a shipment, you will usually see quite a lot of dead fish in the boxes. This can't be helped and is a shame, but is the nature of the business. When you take the fish home and out of the copper treated water, whatever diseases they have will become apparent in your tank