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My window sill planted bowl has measured pH fluctuated from 7.2 to 8.8 pre and post sunlight period. With measured kH around 3, the calculated CO2 is between 3.6 to 0.1 ppm. So Mistergreen's 3-4 mg/l is within range of mine, noting that this is more than atmospheric equilibrium CO2 due to respiration from live stock and bacteria. Note that the water is stagnant and floaters covered 90% of the surface, so surface air exchange is limited. I was afraid there won't be enough O2 exchange, but apparently vigorous O2 streaming during sunlight period generates adequate O2 to carry through day and night.

Homes on gas heat and stove contribute additional CO2. I have a CO monitor but no CO2 monitor to know what CO2 level I have. Elevated CO2 can make people sleepy, and extremely high CO2 can kill by asphyxiation. I think the safety threshold is 3000 ppm for CO2 operated greenhouses.

Indoor air quality is typically worse than outdoor, and better insulated house is worse due to accumulation of CO2 and other indoor pollutants. The worst time is returning home from vacation when normal door opening and closing to exchange fresh air has been absent. It's good to grow house plants and planted tanks for air purification.
 

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Second thoughts....

If Mysiak, mistergreen, and other observant hobbyists are getting better plant growth with added aeration, I propose--after some reference work--an explanation.

In stagnant water, heavy photosynthesis would deplete CO2. It would also fill area surrounding leaves with unwanted oxygen. (Excess oxygen can reduce photosynthetic efficiency in some plants up to 50%.)

Outdoor air contains 350-450 ppm CO2 and indoor air contains more, about 1,000 ppm before it reaches unhealthy levels for humans. So water in equilibrium with indoor air probably contains more than the standard 0.5 and 0.7 mg/l that I quoted, let's say 1.0 mg/l. Not much CO2 but some. (Mistergreen's 3-4 mg/l is a little more than I would predict but maybe possible for those who live in sealed houses, lots of inhabitants, and gas stoves?)

Plants in stagnant water with zero CO2 (depleted by heavy photosynthesis) would probably benefit from water-air mixing that would constantly bring them 1 mg/l air CO2 and quickly flush out the inhibitory oxygen. The associated water movement would decrease the leaf boundary layer speeding up both these beneficial processes.

Aquatic botanists and my book recommend moderate water movement, but I always thought that water-air mixing (e.g., bubbling and degassing) would be detrimental. However, I realize now that in some situations aeration actually could be beneficial.
I need to get your book! I'm still so green (n.p.i.) at all of this, & too busy w/too many other things to do all I want with the aquariums, but after only 3 years in now, keeping fish & wanting to do it right, plus being so admittedly ignorant, I'm glad we have your input available here. So glad! That you yourself continue to research & learn, & then share, is a testimony to your humility as an undeniable & admirable expert. Thank you for everything!!
 

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This was a great read as usual, and gave me a lot of insight as I am figuring how to set up my Ehime 250 on my 36gal. I would never not have water movement on a tank of that size but was prepared to make adjustments to the sprayer bar based on how things progressed and the fact that the general concensus is no or very little water flow and absolutely no filter media. So reading about filters in use with various media at least mechanical is great to hear.
I am thinking of removing the filter from my betta tank but keeping the little bad of mechanical media.

I can verify on Diana's comment on about stagnant water. At one point the pump failed on my smaller pond. I didnt have any fish in it at the time so i figured everything would be ok. They were.... sort of. All of the pond plant with aerial advantage kept chugging along. The strictly underwater plants (cant remember what varieties i had at the time) saw significant slowing in growth, some stopped growing altogether without the pump running. It was small 80-100 gallons so i am figuring this small size amplified the issues i was seeing that might be seen in larger bodies of water. It was probably 6 years ago when all this happened and i think i salvaged what i could into tubs with simple bubblers and most of the pond plants rebounded.
 
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