I can find lots of algae eaters for freshwater aquaria, but these seem
to like unrealistically high temperatures.

I finally found something. The Chinese Weather Loach, also called the
Dojos Loach. They appear to do well in ponds. They're also detritovores,
so if you have a few, they'll eat that extra koi food.

They tolerate a broad range of temperatures, although I don't know if
they will overwinter a freeze zone. I'm in San Diego, so I didn't check
that one out too carefully.

C//