Sorry for the delay in updating things... we are still moving forward with the current hypothesis - cleanser from the air getting into the tank. We have a 10 gallon that's set-up next to the 110. It has a lid and is just finishing up its cycle. We should be able to have another go with a few fish in that tank to see their prognosis.
If they survive in the 10 gallon, we will get the full cover for the 110 and try it there after a few super huge water changes to remove any residual cleanser that might be in the water.
Meanwhile, the 3 cories continue to go along with no issues. They still swim quickly away when my students approach, but they seem to be getting quite used to me. They don't swim away as much when I approach. And, as with any cory, they get very active during a water change.