This 10x turnover is ready about Fish Only tanks and reef tanks it doesn't really apply to any planted tank IMO.
In fish-only tanks, mechanical and biological filtration is the only concern. So you must use filters, not powerheads, to achieve that. Since freshwater fish-only tanks are the most common, manufacturers would logically rate their filters based on that use. To quote a few ratings:
Fluval FX5, up to 400G tanks and 607GPH (1.5X).
Magnum 350, up to 100G tanks and 350GPH (3.5X).
Eheim 2075, up to 160G tanks and 330GPH (2.1X).
And as Diane mentioned, these GPH ratings are quite optimistic, and decrease over time as the media clogs; so they must be derated.
So you're saying you would need, for example, four or more
Magnum 350's on a 100G fish-only tank to achieve adequate filtration?
Quite obviously your claim is false. One will do the job, maybe two if heavily stocked.
How much flow do you need to move dissolved ferts and co2 around a 2 to 4 foot rectangle?
A surprising amount.
CO2 diffuses through still water on its own at 0.0016 mm2/s. Slower than molasses in winter. We can easily distribute it much faster through the tank with just a little flow.
But leaves deflect flow. Plus water is viscous and behaves much like molasses itself directly on the surface of leaves, preferring to stick rather than flow. So there, water is moving at a snail's pace, regardless of the flow in the tank as a whole. If you could see CO2 concentrations at every point in the tank with your own eyes, you'd see a CO2-depleted layer around every leaf, 1-2mm in thickness, where it's being removed by the plants faster than it can cross inwards; due to its own slow rate of diffusion and the much slower flow. It takes high flow in the tank to shrink this layer, known as the Prandtl Boundary.
Other nutrients also diffuse at similarly slow rates, so the Prandtl Boundary affects them as well.
I don't think I've ever had more than 2-3x flow and I've never had a problem with algae or any plant deficiencies. Your mileage will vary based on your setup...
This I agree with. 10X is a guideline, not a rule, and I quoted it as such. Depending on setup, 2-3X flow might work fine. Or it might not. Perhaps you might need to dose more ferts to compensate. But that's an additional issue to figure out, and which could have been avoided. Or more CO2 than normal, same goes, plus it needlessly stresses fish.
When asked for a recommendation
, better to give one that will work best in the majority of cases, not what you or I might have managed to successfully get away with. I have gotten away with lot, even deliberately tested the limits, but that is a topic for other threads.