Ha! I was pretty spot on!
Im not sure how well a heating pad would do to heat a tub up 10 degrees. Im not sure about a tub but I dont think it would do anything to a tank. Your best bet would probably be to put a lamp over it, a desk lamp would do, as long as its raised up enough to not melt the plastic. I figure a regular desk lamp and bulb would spread the heat across the length of the tub and evenly raise the heat inside. Of course monitoring it to find the sweet spot to hit 80 or whatever youre looking for.