thats neat, i think you just gave me a great idea! First off for 40gal...it doesnt seem to have a lot of surface area for gas exchange....so I would try and incorporate a water fall type efect to maximize gas exchange. The main thing i would keep in mind when picking fish is what temperatures is that going to see...being that its outside, if it will see a nice fluctuation then I would go with pond type fish...maybe a couple fancy gold fish 2 or 3 so as to not overcrowd. other than that I would just try and keep it to "friendly" top dwelling fish. If it was me I would do an african butterfly (careful they may jump out) because they eat insects and will eat any that make it into the "pond" and maybe a couple rainbow sharks and a bristle nose pleco. Then throw in some snails and if it was me I would add two 100w heaters, and i bet they would do a good job of keeping it warm when it gets a little chilly out. I think temperature is gonna be your main problem.