Join Date: Jun 2013
Location: Gilbert, Arizona
There are quite a few if you search "Outdoor Aquarium" on google.
I think that the main things against them are that, unlike ponds, they are much less stable temperature wise (less water volume, less insulation from walls/ground), and that since they let light in from all sides, rather than just the top, there's lots of potential for algae if it isn't very well balanced.
If it's done properly it would be a really awesome compliment to a patio/pool area. You've given me a semi-related idea actually! Super-solar algae scrubber!
People are too often merely stating an assumed truth from flawed observation.