It would seem one would need a evaporative cooler to chill air then an
air/water heat exchanger to cool the pond water. It sounds doable.

I saw a recent DIY prototype of an evaporative cooler for an aquarium.
Basically, the guy created a tube into which he dropped water (from the
aqarium) as well as two blowers. Air and water blew down, into the
aquarium, where they bubbled and sloshed. It was very effective at
cooling his aquarium.

C//