I agree with aquariumgishguy if we are talking about a large deep pond, but there is also small ponds of 200gal or less, which are not that deep and you can easely keep tropical fish and enjoy them. The pond I am talking about is aroung 500 gallons and is shallow on the front part, and in other parts its 45cm deep, and I have no problem with viewing my tripical fish and enjoying them every day I see them, feed them and not just keep them.
This is like sort of an external aquarium, not quiete a pond and obviously not quite an aquarium, where there are always advantages to keep these fish in a glass aquarium as well as advantages to keep them in an outdoor body of water/pond. What i am trying to say is that there are lots of interesting options in the hobbie that are not common. I really enjoy sitting in my back garden and watching all the fish outside, and not only in a glass aquarium, and due to the size of the pond I can controll the temperature.