Don’t buy the first fish off the boat.
Avoid buying any new arrivals from your local fish store recently received. If the fish are still in a packing create, that should give you a clue. You should see a large number of bags containing fish floating in the aquariums on the days the store receive new shipments. Most dealers receive new fish at least once a week. Ask your dealer which days these are. A good dealer doesn’t allow customers to purchase fish until there has been sufficient time to quarantine them. This quarantine prior reduces the fish’s stress from shipping and allows the shop’s personnel to treat any disease that shows up in the first few days after arrival.
If you happen to see some fish that catch your eye while they are still in the bags, ask the dealer to hold them for you until a reasonable quarantine period has expired. Most dealers willingly agree to do that. Helpful merchants who take the time to grant such simple requests provide customer satisfaction are defiantly worth patronizing in the future. Stop for a moment and ask yourself one simple question. If I owned the fish shop, would I do this for my customer? If the answer is yes, then you should expect your dealer to do the same thing.