Passafire,
I think you may have done something wrong in your accilmation technique. Or possibly on the way home.
Your fish in a 125g should have been happy as can be. For it to die like that, something was really wrong.
My guess is you didn't drip acclimate it enough. Also what are you useing to measure your salinity? If you are using a swing arm..toss it in the trash and get a refractometer or pinpoint salinity monitor. Trust me, the price of these easily save you the cost of 1 fish.
Step 1 in acclimation.... Float the bag in your tank, time depends on how cold that water got..
Step 2 get a salinity reading on the water from the Fish store...
Some keep them in Hypo Salinity...Mine does meaning SG 1.009
So even if I drip for 2 hrs, the water still is only at 1.015 ish....sometimes i have to go for 6 hrs to get it to 1.026. This would be a good reason your fish died...Osmotic Shock.
Step 3, You know when your drip is done, when you salinity is close or matches you tank.
Also keep a small 5w heater in the bucket to keep the temp stable, the same as your tank.
These added steps i believe will ensure that you have a safe acclimation.
Knock on wood, i haven't lost a fish yet in 1 yr, since i started doing this.