The change in salt level might also be a problem. Also, with shells in your tank, your water parameters (
GH/
KH) would be different from the
lfs's.
I like to do drip acclimatization. This is a prolonged version of the bag & add water routine. Basically, I put the bagged fish in a container, & rig up an airline hose to drip water into the bag slowly, letting the bag overflow into the container. The goal here is to slowly dilute out the bag water with your tank water over several hours. <I usu. add 3-4x the water volume in the bag.>
Sometime, the new fish is just sick to start or the rough catching/handling at the
lfs is the problem rather than your acclimatization.