I'm gonna suggest you stray away from that idea. It would be unique, but far too expensive and not worth it. You'd have to have the tank fully enclosed and rain would get in through any ventilation for the lighting. So it's really not doable unless you build into into a doghouse type of thing but even then, weathering from outside will discolour glass overtime and the heating are one thing as others said.
But just as bad as heating, think about summertime. A 95 degree day, outside with a tank outside, it would be a nightmare keeping that water under 82 degrees. I'd say just put one indoors where people can see it more and enjoy it more. Cheaper, less hassle, just better.