When you purchase a fish for your aquarium, there is no 100% sure-fire way of knowing whether the specimen you’re buying is healthy or not. If there was, there would be no need for fish guarantees! Professional aquarists spend years learning how to recognize if fish are healthy. Fish that appear healthy may have internal problems that are not yet visible. The fish may be genetically weak (which by the way, there’s nothing you can do about), or in the case of marine fish, there are many species which just don’t do well in captivity. There are however, ways to make a well educated guess. There are many things one can look for to determine if a fish is healthy. Many aquarium stores will not assist you in determining whether a fish is healthy or not. After all, they do want to sell the fish! Here’s a list of tips to follow when you’re buying a fish.
Fins should be erect, especially the dorsal (top fin) - Collapsed fins usually spell trouble. Please note: there are exceptions! Puffers curl their caudal fin (tail) up against their body. Butterfly fish only erect their dorsal fins when alarmed. This is normal. If in doubt, call us! We’ll be happy to give you advice at any time.
The fish should interact with one another as they pass each other, moving out of the way when a more dominant fish approaches. Fish that allow themselves to be bullied are not feeling well.
All the fins should be intact. If a fin has a small nip out of it, but the fish acts normally, this may be OK, but avoid a fish with torn or ripped fins.
Respiration should be normal. Compare the gill movement of a fish with other fish in the tank. Although fish with higher metabolic rates respire (breathe) a little faster, all of the fish in a tank will usually move their gills at roughly the same rate. Extremely rapid gill movement indicates stress.
Darker or patchy coloration is a good indicator of stress. Before you buy a fish, try to know what it should look like under normal conditions. Get a good fish book.
As a fish swims toward you or away from you, look carefully at its body on edge for raised bumps or tiny raised growths. As a fish swims past you, look at its fins and on its body for tiny white crystals that look like salt or tiny black nodules that look like "blackheads" (these are parasite infestations). A fish with cauliflower-like growths on the edge of its fins has Lymphocystis (a virus), and is stressed If many fish in a store exhibit these conditions, I would not buy fish from that store.
If a fish bullies or snaps at all of the fish in an aquarium at the fish store, it probably will do the same in your tank! Ask if a fish is abnormally aggressive when buying it. Don’t buy into the claim that an aggressive fish will settle down because it’s the new fish in the tank. While this is true in some cases, sometimes this only lasts until the "new fish" gets comfortable. Then it’s PARTY TIME!
In saltwater, DO NOT BUY: Undulated or Queen Triggers, Sohal Tangs, or Passer Angels, unless you want a very aggressive tank. In freshwater, avoid Manguenese and Dovii Cichlids. The only cure for their aggression is a sharp blow with a hammer, and they’ll probably attack the hammer! If you have a peaceful freshwater community tank, avoid most Barbs, Serpae Tetras, and most Cichlids. Check out the list of beginners-friendly freshwater aquarium fish here: aquariumfishcity.com
Unless you have at least a marine 300-gallon aquarium . . . PLEASE! Do not buy a shark!
Unless you have a 100 gallon or larger aquarium, it is not a good idea to buy saltwater fish that grow larger than six to eight inches in captivity (with the exception of some moray eels). This includes certain groupers and snappers. Exceptions to this rule are slow-moving fish such as Lionfish in saltwater or Oscars in freshwater.
Hope these tips can help someone out there. Happy fish keeping to all!