FWIW, 65 degree water can hold about 6% more DO than 70 degree water and about 8% more than 72 degree water. Not an extreme difference, but not negligible.
In itself the difference between 8.4 and 9.2 is not negligible. It is about 10 %. You are right there.
But is negligible in
hydro, the way hydro works.
At 20 ° C / 68 °F water contains 9.2 milli grams per liter
At 25 ° C / 77 ° F it is 8.4 mg's per liter.
Let's say you have a reservoir of 37 liters = 10 gallons and a plant with 1 kilo of roots.
Then with maximum DO the res would hold (theoretically) 37 x 9.2 mg = 340 mg if the res temp was 20 ° C / 68 °F.
If the res temp was 25 ° C / 68 °F it would hold (theoretically) 37 x 8.4 mg = 310 mg.
Sounds not like a big difference as well
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But it is a difference.
Roots of most plants absorb about 200 mg of oxygen per hour = 3.33 mg per minute per 1 kilo of roots.
Let's say that the res is 37 liters and the roots weigh 1 kilo.
So with the water of 20 ° C / 68 °F the roots would then have oxygen for 102 minutes.
With water of 25 ° C / 77 °F the roots would have oxygen for 93 minutes.
But, and this is important, the moment the roots absorb the DO, the oxygen that disappeared from the water will be filled up again.
Because any movement of the water surface will make sure that air pressure will put new oxygen in the water.
It is a never ending supply of oxygen.
So it doesn't matter if the water can have a maximum DO of 9.2 or 8.4 grams.... it will always be maximum DO and plenty of oxygen (in theory) for the roots.
Only when the water gets too warm, or the res is too small, or you have a huge rootball in a small reservoir, it will be a problem.
But with temperatures between 65 and 79 the difference in DO is negligible. In hydro under normal circumstances, that is.
Keeping your temps low for more DO is useless (again, don't take the risk with temperatures of 30 ° C / 86 °F.
A bigger problem with DO is that DO does not always reach all the roots. That is something that growers should be aware of. Water with 10 mg of DO per liter water is high in oxygen, but if the oxygen does not reach the roots, it does not make much sense.
Some growers have airstones in a bucket of 4 gallons, a huge, dense rootball, and then they see their plant perform badly.
"But I have 3 airstones! And I bought a chiller of $ 300 and my water is 18 ° C / 65 ° F! Should I buy another air stone... a bigger air pump... a bigger chiller?"