New Fish Dies in established tank.

It is definitely due to incorrect fish acclimation.  

Water temperature is only a part of the problem.  Most fish shop employees will only tell you this part.  After the fish get used to the water temperature, they still have to go through the water PH, hardness, and in some case nitrate concentration.   New fish must adapt to all of the above before being released into the fish tank.   Or you are risking killing them with shock.  

Please acclimate them properly.   It saves fish lives.    For details, this article sums it up nicely.   
3 Important Steps to Introduce Fish to new Aquariums