25mm Polystyrene on the back, sides and top would have made an enormous difference to your heating bill, even in an uninsulated shed with your cold temps. Have a look at this heat loss calculator one of the guys on our local forum made;
http://www.fnzas.org.nz/fishroom/viewtopic.php?f=3&t=48031
You can even calculate the cost of the heating if you know the cost per kilowatt hour of your power and work out how much money you'll save with a bit of insulation. I had a quick play around with it for a standard 450g (96x36x30"), based on 19mm glass, no substrate, tank running at 26C and the ambient air temp at 4C adding 25mm polystyrene to the back, sides and top of that tank would have reduced the heat loss from 1279W to 345W.
