I've had outdoor tanks on a couple occasions; I understand the appeal but I didn't find them worth the trouble. Of course, up here in Canuckistan a tank needs to be drained and moved indoors or at least into a sheltered area for the winter, which adds to work involved. But even a year-round long-term tank will have issues and maintenance that exceeds those encountered in a typical indoor tank.
Do you have raccoons? They suck the joy out of an outdoor tank real fast. Hail? I've had it shatter poly and glass lids, and always wonder if it might someday break the tank itself.
And then, of course, there's temperature. Aside from costing an arm and a leg, the heating requirements for an outdoor tank are more stringent than indoors, and the potential for disaster much greater. An indoor tank with a heater that fails "off" will gradually drop down to ambient room temperature; an outdoor tank in cold weather will cool off much faster and much colder. The tank needs to be fairly large to slow down these temperature swings; forget about 20-gallon tanks outside.
The only way this idea makes any sense, IMHO, is to keep fish species that are comfortable in the complete temperature range they will encounter outdoors in your locale, and thus won't require heating. There are more like that than many people realize; do some research and you might be surprised.
Oh, and by the way...even in a shaded area, your outdoor tank will likely receive many times more light than any indoor tank. Be prepared to clean algae...a lot...