So much more info is needed about your plans. Highest summer temp, lowest winter temp, ventilation, any shade trees, heater size, garage size, the list is nearly endless. Temperature control is your biggest issue, in an uninsulated garage you would have to insulate the tank for winter and summer.
I keep all but one of my tanks in the garage which is used solely as a fishroom, the garage is integral to the house so is insulated within the ceiling (there is a bedroom above) and the external cavity wall, the internal wall is solid brick and un insulated but recieves warmth from the house central heating so doesnt need insulating. I have covered the garage door in a thick blanket which is taped to the walls either side and blocked all the gaps beneath the door with heavy blankets to prevent cold draughts, since the garage is the only access into the house with large objects i cannot block the door permamently.
I heat the entire room with a 2000w oil filled electric radiator which keeps all the tanks at a nice steady 25c without the need for any in tank heaters even in cold weather with temperatures as low as -5 over night.