I think for fish is what he's saying is the higher the tank the more need for oxygenation (if that's a word).
For example (may not be a great one) but I like to go fishing with minnows. In the summer they die without being submerged over the boat/off the dock REALLY fast even in a full bucket of water and sometimes fresh flushes of water into the bucket aren't enough.
Now opposite end... I was going ice fishing -- had about 4 inches of water in a 5 gallon bucket of water and my car broke down and had to get it towed back as I got a road from some friendly passerby (before I had a cell phone some years back). Now the car was in the shop for about a week getting the head gasket replaced, shop was around 40 degrees give or take I guess I'm not totally sure but when I went to pick up my car and REMEMBERED there were minnows in the car yet I thought oh great.. stinky car. Nope ALL of the minnows lived in that little bit of water that week, into the next week in my garage until after 2 weeks in that bucket I used them fishing.
Cold water IMO not only slows the metabolism of fish, but also increases the oxygen levels (or the ability to hold oxygen). These same minnows would've been dead inside an hour on a day like today in that same bucket -- but fully aerated with high levels of oxygen.. who know?
Maybe I missed the point but it's late and I'm bored at work and just thought of that story