They still have circulation(some remove the filter is some twisted notion that the reduced NH4=> NO3 conversion that the filter does will help the plants grow better-this does not, the roots and sediments do this much better and have far far more area than the puny little filter when plants are well established- unless you address that, that entire theory is tossed out the window) ...............but some have argued this without ever testing the CO2 critically. It's low in these systems makign much harder to tell what is going on.
Too low for any hobbyists methods of measurements.
So it is still speculation. Growth rates comparisons are also painfully slow since things are CO2 limited. So any data is pretty scant. My take on it is more centered around CO2 mist. If you have mist, whether it's air or pure CO2, or something with a % of CO2, that hits the plants, even if the CO2 is very low, this will help. You cannot use O2 measurements for measuring plant growth except in stiller waters, since that will get blown off rather fast.
I've seen a lot of nice plants doing quite well in streams with waterfalls and current. Not enough to uproot them, but in slower flowing sections.
This tend to do best with lots of mist in the clamer waters below the torrents.
CO2, current, still water has more time to expose the leaves to nutrients?
I'm not sure.
Ole Petersen might know or may have tested something like this.
We have 2 things going on here. One is the production of CO2 from within the system(autochthonous), but this is entirely dependent on O2 from above and the plant's internal production of O2 to the leaves and transport into the root zones-there's that big NH4=> NO3 filter), and this can be a fair amount...............but it is entirely dependent on O2 from above and outside the system and also the O2 produced by the plants.
This is not just about CO2, O2 is used in aerobic respiration to produce the internal CO2, it has to.
This aspect is overlooked but is critical, we keep fish and want a steady supply of CO2.
Adding O2 mist via the air and some CO2 might do better, I'm not sure. Seems that some current is a must for adding O2 to drive enough so the fish do not gasp and the bacteria can cycle the waste. Recall that non CO2 tanks do not get a lot of water changes if any for many months. All that waste must be processed well and at a decent rate.
It takes O2 to do it.
I think some folks have over aerated their non CO2 planted tanks in the past, perhaps a CO2 mist level of aeration, a small mist amount would work better than none or the too much example above.
I'm not sure.
Have not tried and have not measured CO2.
Regards,
Tom Barr