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Purchasing fish for your saltwater aquarium can be a lot of fun, but how do you know if the fish you're buying are healthy? It's impossible to know for certain if a fish is healthy, even if you're a trained aquarist, because ailments don't always show symptoms right away. Also, you can't tell just by looking at it if a fish has a genetic weakness, or is the type to thrive in captivity.

Fortunately, there are a few things you can look out for before making a purchase:

Fins shouldn't be drooping, especially the dorsal fin. A collapsed fin is a bad sign for most fish, although for some species this tip doesn't apply. Puffers normally curl their tail against their body, while butterfly fish have erect dorsal fins only when they are afraid.

In addition, fins need to be intact. Look for nipped, torn, or ripped fins. A few nips may not pose a problem if the fish appears to be behaving properly, but it's a sign to watch out for.

Watch the fish as they swim and mingle with the other specimens in the tank. For example, smaller fish should move out of the way when a dominant fish swims by, as this indicates the fish is feeling normal. A sick fish may allow a dominant fish to intimidate it because it's too ill to move out of the way.

Examine the fish's respiration, comparing the movement of the gills to the other fish. It's normal for some fish to breathe faster than others, but generally fish tend to move their gills at the same rate as those around them in a tank. Quick gill movement is a bad indication because it means the fish is probably stressed.

Other indicators of stress include dark patches and discoloration. If you purchase a book on fish before shopping, it will be easier to determine what coloration is normal for a particular fish, and what's out of the ordinary.

As the fish swim back and forth, search for bumps and raised growths, small white crystals on the fins, or black dots. Fish with these characteristics have parasite infestations. Growths on the fins that resemble cauliflower are an indication of a viral infection and stress. In general, if you notice several fish in one store with signs of stress and disease, don't buy any fish from that location.

And remember, fish that bully in the fish store will most likely do the same thing at home, so avoid purchasing fish that demonstrate aggressive behavior.

For saltwater aquariums in particular, avoid buying Undulated or Queen Triggers, Sohal Tangs, or Passer Angels. Avoid saltwater fish that grow bigger than six to eight inches, unless you're looking to add a slow-moving fish to your aquarium.

Take your time when analyzing the fish so you have time to look beyond the beautiful colors. It's easy to miss common indicators of problems when you have been mesmerized by a gorgeous fish swimming your way. Take a good look at the fish's skin and fins, and watch its movements over a period of time.