Quote Originally Posted by mcavana View Post
the four inches in the middle will be hot, and the four inches on either side will be warm.
OH, perhaps this is where you're confused. If you refer to my original post, I said I have a 20L tank. It's 30"x12". So probably the 4 inches in the middle will be hot, the 4 inches to either side will be warm, but then there are still 9 in on either side for it to cool down further.

That said, I think my best bet is to use 2 UTHs. Since no one seems to have any reason the hot middle won't work though, I'll keep it in mind as a backup.