Big Fish Freak that's awesome. I'm planning to build one, but the weather is freezing down during the winter. I'm watching around for a eco heat, so i can start my own build.
Ddavenmandy, those dimensions are pretty nice to work with. Makes everything nice and simple.
JohnTr, you could always build the tank like an actual wall, and put insulation in to keep the heat in the tank..It might cost a bit more, but the tank would hold heat a lot better.