Interesting points Jim, as I expect to be faced with the same issues.
I am wondering then, how do you keep the mosquito fish population
under control? Or are they self-limiting -- in the same way?

Thanks, David

Mosquito fish get eaten by the koi, usually not intentionally. Mosquito
fish will attack a koi pellet far bigger than their mouth, along comes a
koi, who inhales many pellets at a time. Down go pellets and fresh protein.
I found winter knocked their population down.

I had mosquito fish from the get go, and never had any babies. The year I
decided to continue surface circulation did them all in. I guess it was too
cold, being that they are surface cruisers. The next year, koi babies
survived in the filter. The year after culling my big fish down due to
critical mass emergency, I had koi babies in the filter and in the pond.
And foolish me. I'd also put eggs in plant containers and the lily pond.
Koi babies everywhere. So far I've been lucky enough to give them away. One
ponder in particular took quite a few because they had made a special fry
grow out pond and then didn't have a spawn. Isn't that how it goes? ~ jan

~Power to the Porg, Flow On!~