I showed students the water in the tray by the water filter, and asked students where it came from. The common response was "The tank is leaking!" However, I explained to them that the water level of the tank hasn't changed.
The water actually came from the air! Since the trout tank water is now at 52 degrees Fahrenheit, and the room temperature is about 70-80 degrees Fahrenheit, the water vapor in the air began to condense onto the tube and filter.