But even if you're a more experienced fishkeeper and do your homework right before buying, some fish are still lost that way. Let's consider a few ideas on how to further avoid this.

Bringing new fish home from a store is always stressful to them, and some fish are especially sensitive to any sudden changes, such as Ram's, Clown Loaches and Neons. But for any fish, several conditions can worsen the process to the point of fatality. The most important things to observe are:

General condition of the fish
Healthy fish will stand a much better chance of surviving the stress of moving. Always choose the brightest colored, smartest swimming fish in the group. If possible, check the fish's appetite too. This can sometimes be as simple as sliding your finger on the front glass and seeing which fish follow it around. Of course, many species are healthy but too shy to do this.

Differences in pH and temperature
It is often said that the best water parameters for a fish are the ones it's in right now. This is a wisecrack way of saying that fish do not like sudden changes. A change of more than 0.2 in pH or 2^oC in temperature in a period of less than 8 hours is stressful, and can be fatal. So even though your tank is optimal for the fish, he'll suffer a shock if the shop tank's water is too different from yours. Ask the shop keeper at which level he maintains these parameters in the tank you're choosing the fish from. If you're not satisfied with his answer ask him to do the test in front of you. I once had a problem with some new Ram's dying, even though my water was optimal, and later found out that the shopkeeper kept them in tanks with pH around 8.0, while mine was at 7.0.

Netting the fish
If the shopkeeper chased the fish around for too long, better change your mind. It could be hurt or panicked to the point of dying. Don't feel bad about doing this...you don't owe the shopkeeper anything and it's his obligation to catch the fish without stressing it. I usually get to know the shopkeepers well enough that the let me catch my own fish from the tank. This way I can choose exactly the ones want and net them the way I want.

Transporting the fish home
Make sure the fish has a reasonable amount of water in the bag. Always transport the fish in the dark, it calms them down. I like to wrap the bag in a cotton pullover to keep it dark and help maintain the temperature. If you're going to take more than a couple of hours to get home or if there are several fish in the same bag, ask the shopkeeper to put oxygen in it.

Releasing the fish
Before releasing the fish into your tank, float the bag for at least 10 min, during which you should slowly add your tank water to the one in the bag. This helps adjust the pH and temperature of both waters. After this, throw the water in the bag away and release the new fish into the tank. If you're worried that some of the older fish might attack the newcomers, turn off the lights and/or throw some food in the tank to distract them until the new fish can find a suitable hiding place.

Finally, if after all these precautions, you still lose a fish soon after putting it in the tank, then the only reasonable explanation left is that really, really terrible affliction known among aquarists as MFD - mysterious fish death :-)