One of the biggest lessons that I had to learn the hard way was to take my time when picking out a new marine fish at the local pet store.  It can be very easy to get caught up in the excitement of finally finding the fish that you have been looking for, but you should really look before you leap.  You want to carefully observe the fish to make sure it is not showing any potential signs of sickness.  At times, this can be hard to do if the fish is stressed, but some of the signs to look for are:

• Rapid breathing or rapid gill movement
• Hiding without coming out into the open at all
• Flashing
• Swimming very erratic
• Oddly color spots on the body or small white spots
• Damaged or oddly colored fins
• Loss of apatite

I will typically observe the fish in the store for about 30 minutes.  Sometimes I will repeat this process over two or more different days until I am satisfied there are no visible symptoms and the fish appears healthy.

Once you have had a chance to observe the fish and have not noticed anything odd, I would also suggest that you request the store to feed the fish so you can see it eat .  I would never purchase a fish that was not eating as that can sometime be a indication of internal problems.

And, as always, I strongly recommend placing you new fish in a properly set-up quarantine tank for careful observation.  There are so many other disease your new fish could be infected with that will take a very long time before there are any noticeable external symptoms.

The below links might also be helpful

http://www.reefaquarium.com/2012/picking-out-new-fish-for-a-new-marine-aquarium/

http://www.reefaquarium.com/2013/responsible-fish-keeping/

http://www.reefaquarium.com/2012/quarantining-new-additions/