Yes that is exactly what I did. I T'd off from the top of my hot water heater and ran pex in the attic to the fish room then into a Grundfos motor that sucks the hot water from the water heater through a heat exchange then back to the hot water heater. The aquarium water then runs counter to the heat exchange and it heats up the aquarium. Currently mine is set at 83 degrees and im not using any electricity to heat the tank.
One precaution when doing this. It pulls so much hot water from the tank that you better make sure you have valves on it to stop it down because otherwise the system will syphon all the hot water to the fish room because the grundfos motor is stronger than the pressure in the house leaving you with no hot water pressure in the house. This makes you have to close the valve fully to the fish room to kickstart the hot water to the rest of the house. With the valve nearly closed i have hot water in the house and at the same time there is enough running to the heat exchange to heat the fish tanks.
Good Luck!
I see you've got a motorized ball-valve inline with the Grundfos. Doesn't that stop the hot water from circulating to the heat exchanger when the tank is up to temp? I would think your heat exchanger wouldn't be constantly robbing the house of hot water if your tank is already at the correct temperature.






