ACTIVE ABOVE ALL and good coloration. If you dont get a fish that is perky from the start, they never have a chance of being perky when you take them to your house and stress them on the way there. Good coloring means they are gettting pleanty of minerals and vitamans, and will generaly be more healthy than other fish.
Yes...some are more shy/reclusive....you should research the species a bit before you buy so you know what to look for...avoid fish that are being picked on, cowering in the corner, etc. the fish should be responsive, clear eyes, body not sunken in, healthy looking.....ask the employee to feed the fish so you can see how well they eat....
If you dont get a fish that is perky from the start, they never have a chance of being perky when you take them to your house and stress them on the way there.
I've rescued some guppies from overcrowded chain store tanks that became much more active once they spent some time in my tank with the better water conditions/varied diet. But I just felt really sorry for them, so I wouldn't recommend doing this, especially for expensive fish.
Generaly yes, For example bettas dont move around a whole lot. Find one who plays with its own fins more. Youll know it when you see it. An oscar that sits in the corner of the tank is less desirable then one that kinda cruises around and checks everything out.