Basically, they die of stress due to change of water condition, like kh and ph rate. Sometimes temperature also a reason of the fish die after transfer from other tank/environment. You have to do some homework b4 getting in new fish to your tank.

1. Check original water temperature, try to control in +/-2 dc.
2. Check original water kh and ph rate, if original water ph = 5.5, then do not deliver the fish direct to new water tank which could be ph = 7.0 and above.
3. Don't buy sick fish. Those lfs supply very weak.. check their condition b4 buying.