I would venture a guess that it was the water parameter change that did it. Many people think that PH and temperature is what is involved in acclimation, but I feel it is a bit more complex, involving TDS (total dissolved solids) of which RO water basically has none and hard water has a lot of... I am surprised that a LFS would have fish in water that is not like what most people have at home, knowing that most folks probably just use treated tap water. They risk making lots of people angry when things like this happen, not to mention the catfish's well being!
I am in agreement that a much long drip acclimation would have helped your catfish here.
I bought 3 Sterba Catfish from my LFS yesterday...this morning they were all dead. At the store they have them in RO water and seem to be doing fine. My tank is more on the hard water side but acclimated the new Sterba's a little over time by pouring water from tank into the bag before releasing them over a 30 minute period. Everything I have read about Sterba's is they are not to be in extremely soft water because it can damage thier barbels. I have never had new fish die like that ever...I even put them in my most successful tank with other cory catfish, 4 danios, and 1 borneo sucker fish. Any idea what could have happened to cause them to die? Was is the RO to harder water that did it? By the way, the fish store gave me store credit to reimburse me for the loss.
