Another way to keep down the water heating energy is having a lesser WC. It takes a lot of energy to heat up cold pipe water. So I guess the denitrator is a part of the energy saving plan. And ofcourse having styrofoam 3D background on the 3 sides of tank has been helping a lot in containing the heat.
That's the heat exchange system I was talking about. For the aquariums you run a pipe through the boiler or something. I am not yet familiar with the system, but it's what Mike does (MFK).