I get asked quite often, especially by beginning aquarists, what it is they should be looking for, or at, when purchasing a new fish. I was thinking that now was as good a time as any to put together a list of suggestions.
1) Look at the general condition of all the animals in the tank. Granted, any LFS is a stressful place for a fish even under the best conditions, but they should have decent color, free of patchy areas. They should look well fed, but not bloated. If you have any doubts, or you’re working with a notoriously fussy species, ask to see them eat. Also check how they’re swimming. The fish should be able to move about freely without struggling.
2) Check for the common communicable diseases and parasites such as ich, velvet, fin and tail rot, parasitic worms, pop eye/eye cloud, and any blood streaks in the fins. If you see any fish in the tank with any of these problems, then ALL the fish in the tank are suspect as well and none of them are safe for purchase. If the tank in question is part of an integrated system and not isolated, then the fish in all of the tanks in that loop are also suspect and the fish are not safe for purchase. For those who might not know, an integrated system is one in which all the tanks are connected to one central pump so therefore share the water between them and also any pathogens or parasites. If you’re not sure which system it is and you spot a sick animal, be sure to ask before buying fish out of any of the tanks.
3) Ask what pH and temperature the fish are being kept at. While this may not be critical for some species, for other it is. Cardinals, Rummynose, Rams, Discus, Clown loaches, and Bleeding heart tetras are just a few examples of fish that can be sensitive to both pH and changes in pH. If they don’t know what these basic parameters are, ask them to test it for you. If they can’t test it, seriously question the quality of any fish there.
4) Make sure that the species your considering purchasing is compatible with you and with it’s prospective tankmates (if any). The best way to accomplish this is to do your homework before you purchase that new species. Unless you know the person selling the fish pretty well and are completely confident in their knowledge, don’t take their word for it, do some research. Come to think of it, do your research no matter what. With the internet around, it’s fun, it’s easy, and as much as I hate to admit it, all of us can make a mistake once in awhile. Never, ever trust the little info cards hanging on the tanks, they're wrong more often than they're right.
5) Watch the person catching the fish. Any competent LFS personnel should be able to catch any given fish in the store without having to run it down to the point of exhaustion, the entire process shouldn't take more than 30 seconds. If they do, tell them you no longer want that fish. The upcoming transfer to their new home and environment is stressful enough, you don’t want to start out with a fish that’s already stressed by exhaustion.
A discus is like a pretty girl wearing REALLY expensive jewelry. "Wanting" is often a far greater thing than "having".