Sounds like it could be your acclimation process. Do a search on acclimation..
Drip is the best but floating can work as long as you take you time.
Check the salinity of the LFS
water before you start and then yours.. If your floating take out some LFS
water and then add 1-2 shot glasses of your tank water float for 15-20 and repeat this for at least an hour... Check salinity of the bag and if it still does not match your tank water then keep going..
Another thing that can kill the fish is a big jump in PH
so you may want to check yours and the LFS
ans see how they compair.