This probably goes without saying...Water conditions in your holding tanks should be monitored for optimal performance.
But, the water condition of the bagged water, or in your case...the 15 gallon tub, may be just enough different than your holding tank to cause some stress. Water conditions in either may be within acceptable limits, but on the opposite ends of the range. My thoughts are that a lot of fish in a small container for extended periods will develop high ammonia levels, but not enough to kill. Once dumped into a holding tank with much lower levels of ammonia, the fish can be shocked by the difference.
It is beneficial to temperate acclimate (temper) the new fish as Snipe suggested AND gradually add some of your holding water to the new fish over the course of the acclimation time.
I have acclimated fish by placing the fish into bags or buckets 2/3 full of travel water, placing the containers in the new home for temp acclimation, and every 5 minutes add about a 1/5 the container capacity of new home water for about 20 minutes total. The only exception is if the fish start piping at the surface indicating that they are suffering for O2...adding new home water faster can help with O2, but may over-speed the temp and water condition portion of the acclimation. It can be a dance, but you get the idea.
I am not saying this IS your problem, but it helps with transfer for sure.
