Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
All the stresses of acclimation as suggested above can certainly happen, also if the fish have any low level infection the stresses can make it worse so they can die of whatever they are infected with. The worst part of these stresses is water with higher or lower mineral and salt level than they are accustomed to.
Test the GH, KH and TDS in the bag. Even better, go to the store ahead of time and test it in their tanks. Explain that you have had problems acclimating the fish and you are setting up a quarantine tank.
Set up a quarantine tank that matches those parameters.
When you know you will buy fish within 24 hours add some cycled media to the filter, and some plant cuttings. Let the plants drift in the tank.
When you bring the fish home acclimate them to the Q-tank. This should be really easy, it is very similar to the water they were in at the store. However, to make it even easier on them you can drip acclimate if you feel you need to.
While the fish are in quarantine watch for diseases and parasites. If they need any medication, do so.
Also, gradually change the water chemistry in the Q-tank until it matches your display tank. Most important: I make the TDS match the store water by starting with the GH & KH. One store near me adds salt, so I do too, when I buy from that store. Over the quarantine time many water changes lower the water hardness, and remove the salt. The fishes bodies change to be able to deal with the changing levels of minerals and salt. It takes a while for them to change like that, it cannot happen in just an hour or two.
Time in quarantine will be at least 1 month past the time that any medications were used.