Sherry, it a way, it's fairly true.............

A non CO2 method works well and the KH is used, the bicarbonates..........
However................

That assumes that the plant can use bicarbonates as a carbon sources, most of the 300+ species or so cannot. Instead, they adapt to low levels of CO2.

Non CO2 tanks quickly remove the KH, then the tanks are well run from then on.
How is that possible?

I agree adding KH is not a bad method for carbon, but it's painly slow and much harder for plants and algae to use.

You can also use KH and CO2 together, but plants will use only the CO2 until it gets very low, then switch to KH, if they have that ability.

So what does that say about preference, growth rates?
CO2 is much better.

The other issue that might be happening, the tank may be Ca or Mg limited and adding aragonite adds more than just KH........
I've used arag in several Rift set ups with plants, I think it does work well.
Look no further to Onyx sand, which is also labelled Grey Coast calcite sand.
Same base materials, different crystal structure(arag vs calicite).

So why wouldn't adding Onyx be better than arag?

Regards,
Tom Barr