it would probally be better having an above ground pond with a 'radiator' or heat exchanger buried under ground with a pump connected to a thermostat to pump water through when its cold, and turn the pump off when its reached temp. hmmm makes me want to move to rotorua.
or use ryan jurys idea, kinda, and get a insulated shipping container and seal off half of it and full it with water. instant monster fish tank!