Where would the heat mat go, though? Under the gravel?
I would love to find a heat that could wrap around the filter, or even the hoses! I wonder if that would work. Actually, if you took the downward/intake hose from the tank and extended it a couple of feet, you could probably figure out some way to coil it so that it's in the heated area for a while without weakening the flow!
"The fewer our wants, the nearer we resemble the gods." — Socrates