Short video showing the pond. It is roughly 27 feet long, averages 2 feet deep, and is probably 4 or 5 feet across on average (the narrowest section is 2 feet, widest is 6 feet). All the larger fish are 22 years old, the smaller ones are babies that have made it (according to the former owner who built the property). I think there are 17 total fish at an average length of about 1 foot.
My wife thinks there are to many fish for the amount of water. Neither of us really know anything about Koi, the owner gave us a tutorial on how often to feed them, the fact they hibernate if the water drops below 50 degrees, how to clean the filter, where the extra bubblers are, and where the thermometer is. We discovered on our own that children love feeding them and so far are really enjoying owning a Koi pond, but we just want to make sure we're being good owners and the fish aren't stressed.
We looked for signs of stress (spending to much time near the surface, spending time near the waterfall or other sources of oxygen coming in, rotting smells) and none are present but again, we don't know what we don't know. Reassurance is just as nice as advice.