Question
I was wondering if you know if I can add 4- 4"goldfish to a 5,000 gallon koi
pond that holds currently 6- 12"+ koi fish???  Also was wondering if it is
nessasary to run the waterfall filter all the time since the pond is so large???  any
info on this would be greatly appriciated... there is too much info out there and
it varies so much that I wanted to ask an "expert"   thanks for your time, crystal

Answer
Koi and goldfish can for the most part happily co-exist.  There are a few downsides to consider.

Anytime you introduce new fish, they might bring in something bad like parasites, bacteria, funguses, etc.  It's a good idea to quarantine.

Larger goldfish and koi will eater smaller goldfish and koi.  Goldfish especially love to eat eggs and fry.  Since they are usually more prolific, it's more likely that goldfish fry would survive than koi fry.  I have a pair of mature koi.  I'm pretty sure they're male and female.  Yet, I've never found any baby koi.  If you add goldfish, you will get fewer baby koi which can be a good thing if you don't want a ton of them.  Since goldfish breed so well, if your pond has plants especially, you may get more than you wanted in a few years.  My pond's goldfish population has been controlled by many species of predators.

The waterfall filter should be run to provide mechanical and biological filtration and aeration.  While the animals could do without mechanical filtration, they really need the biological filtration to prevent an increase in ammonia and nitrite.  More importantly, aeration is very important for koi (and goldfish).  If you turn off your waterfall in the winter because you're in an extra cold area, I suggest adding an air stone to the pond to keep up the oxygen levels.  Pumps and filters should run all the time.  Oxygen levels in the summer are lowest over night and at dawn. That's because any live plants or algae produce oxygen during the day but use it at night.  If the pond doesn't have any plants or algae, then the oxygen levels will probably be lower during the hot part of a hot day.  In the winter, with ice over most of the pond, oxygen levels can go below levels that can sustain life so aeration or water movement is important.

Good luck!