I believe the problem may be your acclimation procedure. Floating the bag in the tank and then just releasing the new fish is NOT acclimating the fish to anything except your temperature. It is possible your fish are shocked from all the various differences in water chemistry between your water and the LFS water. You need to slowly acclimate them to all these changes in chemistry.
Personally, I have never ever had a fish die within a few hours of getting it, and this is how I acclimate: Float bag in tank for at least 15 minutes, as you've done in the past. Then, add about 1/4 cup of water from your tank into the floating bag. Wait 5 minutes, add another 1/4 cup water from your tank into the bag. When the bag is full (or too full to float) slowly pour out about half the water into a bucket, and put bag back in the tank to float. Again, add 1/4 to 1/2 cup water every 5 minutes or so until the bag is full. When the bag is again full, net the fish out and release into your tank. Never let the LFS water out into your tank. The whole process could take an hour or so.
I know many others do similar acclimation procedures, and have also had great luck keeping even sensitive species alive. I'd really recommend you take the time to acclimate this way, and I think you may stop losing fish so quickly.
Personally, I have never ever had a fish die within a few hours of getting it, and this is how I acclimate: Float bag in tank for at least 15 minutes, as you've done in the past. Then, add about 1/4 cup of water from your tank into the floating bag. Wait 5 minutes, add another 1/4 cup water from your tank into the bag. When the bag is full (or too full to float) slowly pour out about half the water into a bucket, and put bag back in the tank to float. Again, add 1/4 to 1/2 cup water every 5 minutes or so until the bag is full. When the bag is again full, net the fish out and release into your tank. Never let the LFS water out into your tank. The whole process could take an hour or so.
I know many others do similar acclimation procedures, and have also had great luck keeping even sensitive species alive. I'd really recommend you take the time to acclimate this way, and I think you may stop losing fish so quickly.