Gorgeous fish. =]
About the temp. Does it suddenly change temp or is it a very gradual change? Alot of changes in temperature can cause alot of stress on the fish.
I see his tank is by the window. Usually sunlight during the day warms the tank, leaving it to get cold at night.
If you want to keep the tank at a good, constant temp, I suggest you invest in a thermometer & a small heater.
How many gallons is that tank?