Hmm.. I second what BlueIz said.
With 80% or so of the old water still in the tank, kinda hard to alter the temp unless you were using Frozen or Boiling water. When filling with a Python, I usually can feel the temp and compare with my finger (Tank - Tap) and adjust from there. I also add my water conditioner (I use API's) while I fill. I find it dilutes faster while there is water movement from my Python. So here is what I think could have happened. Depending how long your tank had been running with a Bioload in it, perhaps when you pulled the Python out of the water and the water in the Tube part falls back into the tank, it may have disturbed the gravel and release / polute the water causing a Ammonia Spike and the fish being directly affected by this. Now, this may not be the case, but JMI.