As the temperature decreases, the oxygen gas equilibrium for water actually increases. At first blush it’s somewhat non-intuitive, here’s a graphic:
[1]
Along with the slowing of biological growth (plant and microbes), lower temperatures play a large part in the available DO. Though, going too cool slows and stresses the plant.
Going warm means less DO. You’ll need an increased amount of circulation and an increased amount of air contact. So, a bit of a balancing act.
There is a whole line of thought on aerobic vs anaerobic biology that extends from this as well.
Oh, and to point out, just because the dissolved oxygen graph says something doesn’t mean you are actually achieving those numbers. You have to have a way to ensure that oxygen is being introduced into solution faster than it is being consumed (by the plants and other aerobic biology).
Ditto. That appears to be the consensus temperature for the typical DWC. < 21C and > 17C