Great reply, thank you!
"dr"green, I will wade on in here to say that one solution does not fit every growers situation.
Where I live heat is always an issue.
Flumming with even a small waterpump can put a borderline temp rez over.
I run a nice pond sized air pump that is mounted in the fridge.
It pumps a nice chilly air through my potentially bacteria laden airstones, which in turn helps me to keep my temps in line in separate rez's without the need for a costly chiller for each rez or trying to cool them all somehow with some other common source.
The idea that airstones put O2 into the water with their bubbles, is based on a fundamentally flawed understanding of what action the bubbles provide in your solution. They do exactly what flumming does, they raise the lower water, to the surface where it absorbs O2 and then follows the current down as new water is forced to the surface with the rise of the bubbles.
Now , it is true that bubbles that are small enough to dissipate, before reaching the surface will force O2 into the solution, but those stones tend to be 10-30 times the cost of decent "pond" airstones.
My one air pump can drive 10-15 separate 20l -40l rez's.
Separate waterpumps can certainly be more costly to purchase and maintain.
In hard water solutions airstones will clog eventually but can generally be scrubbed back to usefulness with relative ease or replaced quite cheaply. Waterpumps will suffer from the same accumulation of minerals and become clogged, but they are much more difficult to clean and more expensive to replace.
Now if I lived in a cooler climate that require me to heat it, waterpumps and flumming might be a part of my hydro regime.
And I might add, I have a great deal of hands on experience with everything from nearly passive waterfall systems to heavily refrigerated and recirculated systems in the tropical fish industry.