Synopsis.


1) Get rid of the many fish -> like said before.
2) Remove all pumps (or just shut them off)
3 Except one pump that would directly feed the waterfall. That one would just be a pump in a housing, a big hose to the waterfall, done, zero maintenance.
4) Get 10 goldfish, small ones. Warning: they breed (really Wide-O? )
5) Get an air pump. A decent one. Get a Hi Blow Air 80 (ltrs per minute), a plastic distributor (so you can hang 10 small hoses on it, some transparent hose (10 or 20 meters) and a few air stones that connect to those hoses. (no, I have to financial interest in air pump brands LOL, but I dare to show this recommendation to every koi owner in the world) See picture for how it looks (contains wrong brand of pump, but shows you what you would end up with)

just focus on pump/distributor/hoses



Hoses attach to stones like these:



6) keep that running all year as long as temps are > 12°C
7) may need maintenance every 2 years, which means open up and install new standard part (the "vibrator" for want of a better word) of the air pump (which, for all clarity, is out of the water in a dry place).

And you are done.

Zero maintenance, happy goldfish. Cost? About $200 for air pump/hoses/stones. Never ever again wade into the pond.