Why do some of my fish die when I introduce them to my tank

I have a 55 gallon fresh water tank all non-agressive, community type fish. The water is fine .no problems there. The temperature is right on. Yet, when I go to a PetCo store and buy .. say, a Goldburst Pladdy, and one other similiar kind of fish .. one of the two will die. I leave the bag in the water for 20 minutes as told .. before I release them .. right now, 2 from last night are alive, but one is at the top of the water .. staying in one place and did not swim over for food like the others do .. so I assume this fish will die soon. I have a small neon tetra in the tank as the "canary" to warn me if something else might be wrong .as they tend to die the easiest .. he has been in the tank for 2 1/2 months .. no problem.. no problems with the other fish .. just new ones .. one out of two .. or two out of 3 new ones may die . Any ideas as to why this is happening ? Thank you .

I always add aquarium salt to the tank when adding a new fish. Before adding the new fish to the tank, you should add a cup of your own tank water into the bag, wait 20 min and take a cup of water out of the bag and add another cup of tank water back in. Do this three or four times to slowly acclimate the new fish to the specifics of your own tank. Don't feed your fish the day of and the day after a new fish. It will give your beneficial bacteria a chance to catch up with the new additions. It doesn't affect your original fish as they are not suffering from the stress of being shipped from the supplier to the pet store and then to your own tank.