Answer: Convection
Explanation
(a) It is not conduction
Water is not a good conductor of heat because it has large thermal mass, and it does not sustain large temperature gradients.
(b) It is not radiation
Water undergoes phase changes at 0°C and 100°C. An aquarium does not run at these temperatures. Between these temperatures, the driver for radiative heat transfer between different temperatures in the aquarium will not be significant.
(c) It is not evaporation
Ambient temperature around the aquarium will probably be in the range 20°C o no more than 27°C. The temperature difference between aquarium temperature and ambient temperature will be insignificant, and cannot heat an aquarium.
(d) It is convection
As regions near the heater rise in temperature, convectional flow currents drive warm water toward cold water regions.due to diffusivity. Although it will take some time, the tendency for water in the aquarium is to move toward an equilibrium temperature.
