Why can temperature control cost more in an open aquarium system?

In an open aquarium system, water flows through the enclosure only once before being discarded. Because the system does not recirculate the water, it requires a continuous, high-volume supply of new water to maintain the environment.1

Temperature control becomes costly in this setup because the entire volume of incoming water must be constantly adjusted to the specific thermal requirements of the aquatic organisms before it enters the tank. Unlike closed or semiclosed systems, which recirculate and treat the same body of water, an open system forces the operator to expend significant energy to heat or cool a never-ending stream of fresh water.1