Wait... Is the tank going on the inside of the house, viewable from the garage? Or in the garage, viewable from the house???
If it is going inside the house, viewable from the garage, then I would just seal it with some caulk and be done with it.
If the tank is going in the garage, viewable from inside the house then I would insulate the sides and back. Not to mention bottom and stand. Eventually including extra heaters, depending on your climate. Also, in this case, I would recommend using an acrylic tank as they insulate better against the cold.