Isn't that the base reason that Walstad promoted the Siesta period for low-tech dirt tanks? Co2 that was built up overnight would be depleted during a full day light cycle. By doing the seista period it allowed co2 levels to be built up again. The exact decline or increase isn't really that important. What is important is that it made more co2 available.
First the siesta was not to my knowledge proven effective.
Second, Walstad isn't that dirted, no water changes, and even the filter is optional? (aka all Natural....)
Walstad can promote whatever she likes, it is up to us whether we beleive what she has written or extrapolated is applicable to the tanks we discuss here. I certainly don't in this case.
Not sure I understand the question, or what PH or that pond in the picture has to do with It? Plant uptake can easily zero out 2-3 ppm of CO2 in an aquarium. Have I ever tested this personally with equipment sophisticated enough to read a ppm of CO2? No I havent
That was the example from Walstad's book of a low alkalinity lake, it happens to be chock full of algae, phytoplankton, and plants that can use CO2. This lake allows CO2 to go to practically zero (ph ~10) and great fluctuations in pH due to low kh. It is so different from the conditions in our tanks I can just about disregard any conclusions drawn from studies on this lake as being applicable for hobbyist purposes.
It is my assertion that shallow(small) tanks never go to 0ppm CO2, there an equilibrium maintained with air CO2 concentration and thus the impact of Carbon uptake from Carbonates is negligible. I think this is an easy assertion with reasonable flow from a filter and surface mixing.
I still remember the debates with @Edward
about pH decreasing 'dramatically' at night from all the CO2 plants were releasing. It has never happened to me or to him or anyone I spoke to who runs CO2 24/7. The same can be said about pH increasing dramatically during the day from depletion of CO2. I have never seen it in a hobbyist tank and would welcome someone with data who has.