Winter: insulate the garage, add more insulation to the tank, and run heaters on the tank. Rugs can help insulate the floor if it's concrete. I would not recommend using space heaters to the garage or routing HVAC there unless you have plans for the garage to be something more than it is.
Summer: chillers, aerators, bags of ice, fans, run the water uncovered where possible to encourage evaporation, add water routinely. Block off light into the garage during the day to reduce heat accumulation and consider using large fans to vent heat from the garage at night. If the floor is concrete, it's likely the coolest part of the garage, so remove the rugs.
Of course the species need to be capable of withstanding some high temps and low oxygen conditions if the temps get too high, and they also may need to be able to withstand low temps if they get too low.
Feeding regimens need to accommodate different metabolic rates if the temps vary too much.