So my friend got back to me, and this is a really interesting topic! I'm going to try and do my best to transcribe what we talked about here
He started off by making a great analogy which I think many of us will appreciate: When you pour yourself a beer, how do you make it so that you get the least amount of foam? That foam is effectively oxygenation, so pouring a small stream along the side of the glass slowly will result in the least foam, and pouring your beer from a great height vertically into your glass will cause your entire glass to overflow with foam. More foam = more oxygenation.
Then he went on to say, that oxygenation in your water is basically the result of disruption of the water, as well as a factor of the increased surface area caused by the disruption. He also said that noise and disruption tend to follow each other with water, so optimizing for oxygenation also has the potential to maximize sound, but he did have some thoughts on effective ways of disrupting the water while not maximizing for noise.
The first thing he recommended which would maximize the oxygenation by maximizing the surface area of the water but would still be a somewhat noisy solution is to try and make the stream of water as wide as possible, and to break it up before it hits the water. He made the comment that small rain droplets hitting the surface of water both have maximum air exchange with the droplet itself (because it's a sphere), and the individual droplets hitting the water surface cause surface disruption which also causes air exchange to occur at the surface of a tank. He said that you should put a showerhead at the outlet of your filter and have that trickle into the tank
On a more realistic note (after I shot his showerhead suggestion down) he said putting something like a rock or wide piece of bark or driftwood as a ramp between the current filter outlet and the surface of the tank will cause eddies to form, which are apparently a key for getting air and water to mix. Apparently, slowing down the flow somewhat and having a bumpy surface will help generate the eddies.
He said the worst approach would be to have a "faucet" approach where a narrow stream of water hits the surface of the aquarium, because even though this causes some surface disruption it's not as much as would be possible by either angling the flow or widening the stream. I challenged him saying this seems to go against his beer analogy, to which he retorted two examples: 1) if you have a half full beer glass and you pour more beer into it straight on, your beer may foam a little but it will remain in the glass. 2) if you dump your solo cup on the ground at a party, the entire area where you spilled your beer will foam up briefly.
Finally, he also said that an air stone would help with oxygenation, because it would increase the surface area of the air relative to water under the water surface, and he continued on to say that smaller bubbles that move more slowly will maximize the surface of the air in contact with water (but doesn't have much effect on the surface disruption of the top of the tank).
He did give two caveats for the entire conversation we had: he said he's not a chemistry person, so the effect of the oxygen gradient (air -> water due to the presumed decrease of O2 inside the water due to the fish) isn't incorporated into any of this, so some of these effects described may be more/less strong due to the effects that the gradient has in terms of how willing the water is to "pull" oxygen into itself. He also said that depending on what the water flow inside the tank is, it can also have effects on air exchange at the surface of the water ("poorer" water may be more willing to pull air into it, vs if the oxygenated water all just sits at the top and doesn't mix in with lower layers of the tank, those aspects may be reasons for/against direct injection of oxygen into the water).
@AbbeysDad I want to loop you in to this conversation as well, since it builds off the air stone conversation we were having the other day.
Hopefully this helps!
