We once got the pond down to one goldfish.
(We've had two fish adoptions where we gave
away fish.) From that one goldfish and four
introduced koi (to take care of runaway snails)
we have had many generations and I see baby
fish all the time. The other day I noticed a lot of
damselflies laying eggs in the hornwort. Whenever
the water level goes up the fish spend a lot of
time policing the edges hunting down insects.
We seem to have gnats hatching from time to
time. Snails reproduce in the waterfall pool and make their
way into the pond where they don't last very long.
In my fishless ponds I'm always amazed at what
shows up - rattail maggots (the larva of the dronefly)
is one of my favorite discoveries. I'm sure that those
wonderful critters get eaten in the fishpond.
The fish seem to spend a lot of time mowing substrate
algae and the critters that live therein.

k :-)