What I said on the other forum is that most people talk about a 20 gallon because it used to be (and still may be ) easier to come by than the exo-terras. Sometimes people see "18x18x12" and they accidentally read it as "12x12x18" so of course they think it's not a good idea. There's a book by Ray Roehner (who used to own a pet shop specializing in Leopard geckos) called "The Leopard Gecko Advisor" who actually recommends the 18x18x18.