I don't think heating is really a big issue with tanks. It's usually humidity maintenance that gets people pulling their hair out. Insulating with that wouldn't hurt anything, and might help maintaining steady ambient temps, but if your room temps are low enough that you believe you need to insulate, you'll still need an overhead light on a dimmer. Which means you'll still be battling humidity Give it a go though! Let us know how it works out / cost effectiveness vs foamboard etc