I'm sorry to announce that we have had a catastrophe with the fish. I use filtered drinking water to change the tank water every other day. I bought the water at a different-than-usual place yesterday, and changed the water this morning. We believe that there was something in that water that has resulted in... well, we have lost Chris and Godot, and Quille is very, very weak.
I have pulled the tank apart and cleaned and rinsed everything, refilled it with the proper water, and changed all filters. We will see how many fish we are buying tomorrow.