Koi died while floating

Question
I recently build a small pond in my basement.  I filled it with water from my larger outdoor pond. Then proceeded to bring some of my smaller koi indoors.  In total I brought in 8 koi.  I floated each inside its own gallon bag, and all but one were perfectly fine. Also note the outside ponds temp is 67f and indoors was 71f, not that much of a difference.  After floating for 20 minutes one of the koi just floated to the top of his bag, almost like it became paralyzed.  It wasnt gasping and there was plenty of water for that amount of time and size of fish.  I brought the koi back to the original pond and floated him in there. Another 20 minutes passed and he was swimming around the bag again, so I let him go into the outdoor pond.  The next day I figured I would try again with him, so went through the same process of bagging him and floating and again he did the same thing after 15-20 mins. So again I returned him to the original pond to float.  Unfortunately this time he never came back from that state and died that night.  I know the temps of both waters weren't too far from each other and the water chemistry should have been the same. Even though he never left his bag. I also remember that koi can handle an increase in temps much better than a decrease.  Also he was one of the last ones I brought in so if there were any chemicals in the food grade bag wouldnt it have effected the others?  Im so confused and angry at myself for bringing him in the second time.  I was wondering if youve ever heard of this before or experienced this?  I feel terrible.

Thank you for your time,
Jim

Answer
Hey Jim, moving fish from one tank to another is pretty traumatic, thats why  I never do it. Even introducing fish to new aquariums is troublesome. We I get new salt water fish I let them acclimate for 2 hours and have a small drip running in the bag and still sometimes they don't make it.. Its like taking you right from where you are and and dropping you in Alaska. You are bound to lose some fish usually the weakest. Thanks Eric