It's possible that your tank is the shade, or that 21 C is the average temperature throughout the day. Remember, water has a massive specific heat compared to air. At nighttime, when air temperature drops by several degrees, the water may only be 20 C, for example. Water needs a whole lot more energy to heat up and likely doesnr transfer very much heat to the air around the tank at all. While the air temperature cycles throughout the day, the water temp has a cycle with smallwr amplitude.
