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No problem here. I actually have four 40 gallon tanks and only one has the problem. Two of the tank I use a dimmer light throughout the day with a “burst” period of bright light. This tank has just one very bright light on for 12 hours. It’s starting to get green spot algae so last night I switched it to the siesta method in hopes that the pinnitifida will grow without issue. :). Hopefully the buce appreciates the increased co2 as well, but it has been growing quite vigorously without it.
Because this 12 hour daylength has sparked so much controversy, it was good to get your information.
In nature, yes, plants get a 12-16 hour photoperiod, BUT the light is dim during dawn and evening hours. They do not get 12-16 hours of continuous intense light as they would in an aquarium.
So a natural 12 hour daylength is not the same as an artificial 12 hour aquarium daylength (same high intensity all 12 hours).
You have set up 2 of your tanks with a more natural daylength. What is the daylength for the two tanks that you have set up with dimmers?
For the problem tank, I think a Siesta is a good idea or maybe just duplicate whatever works in your other tanks.
 

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I'm curious what you use to measure co2, do you just use a drop checker? I think it would be very interesting to have similar co2 data to what you posted about siesta lighting but from a tank with fluctuating lighting intensity. I wouldn’t be surprised if co2 levels did not drop as quickly and persisted longer throughout the day before dropping off completely. My hypothesis would be that the siesta method ultimately would provide the greatest amount of co2 for plants on a daily basis, but some concrete data would be interesting.
I used a drop checker. The LaMotte test kit. It worked very well.

Folks, I do not think that just measuring pH translates into measuring CO2. If this were true, everyone would just measure pH, and that's not what municipal water treatment plants do.

CO2 is a fixed % of bicarbonate at each pH. For example, if you have a pH of 7.5, CO2 is ~5% of the bicarbonate concentration. At any given pH, if you have a high bicarbonate concentration, you would have more CO2 than if you had a low bicarbonate concentration.

To calculate the CO2, you need to know the pH and the KH (i.e., bicarbonate concentration). Many aquarium plant books have a chart that you read off the CO2 based on the KH and pH in your tank.

pH changes during siesta may be small or none at all if the water has a high level of bicarbonates. (Bicarbonates buffer the pH.) I got big swings in CO2 conc. due to Siestas in my tanks, but very little pH changes.
 

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I just measured KH early in morning and late in afternoon on all 9 of my potted plant tanks. The KH did not change. Surprising. However, it reminds me of when I did CO2 measurements with the titration method. The LaMotte test kit was very sensitive for diel CO2 changes, and provided the data for my Siesta Regimen graph (Fig XI-2 on p. 179). At the same time I was doing this study, I also measured pH in the tanks and bowls. There was almost no change in pH via colorimetric tests.
My Take: KH and pH are too crude or too insensitive to provide accurate information on CO2 in Low-tech tanks, especially for something as sensitive as diel cycling.
Bottom Line: I would stick with the titration method for measuring CO2 in low-tech tanks. This is a direct measurement of CO2 that worked incredibly well for me and was sensitive enough to show real changes in CO2 during the day.
 
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