Insulation is no longer just for your house. Many people have experimented with it for backgrounds for tanks and have come up with some very cool backgrounds. Insulation is easier and not as messy as great stuff, it is very easy to carve and once covered holds up well.
Here are some photos of one of my favorite tanks by Arielelf on Dendroboard.com
Here is the same tank more grown in:
And here is a close-up of the waterfall down the side:
Here is another tank that used insulation sheets as a background created by Finchfrogs15 on Dendroboard:
Have you ever created a background out of insulation sheets? How is it holding up?