you could always build an enclosure/closet around the back of the tank, with insulation and vents into the living room, so that air is taken and circulated from the house, instead of the garage... you'd loose a little room in the garage, but would still have wall space to hang your badminton rackets and gardening tools on...
this is exactly what i was going to suggest, you could build an enclousure around the back of the tank it hsould keep the heat confined and also provide a barrier so no nasties from the garage will get in