Fishie are you saying that your garage door is pretty much open 24/7?
If so it would help A LOT if you could keep it closed as much as possible...
Your tanks heat up a small layer of air around the sides of your tank and that helps insulate against heatloss.. This works best of course if there is no movement of air in the room, but as soon a little wind blows in this layer is removed and you loose a lot more heat from those surfaces..
I'd say keep the door shut as much as you can and insulate with 25mm poly on whatever sides you can.. best you can do.