How much CO2 is too much? (NO fauna)


New Member
6 Oct 2019
Greenford, London, UK
I've just set up a new tank and haven't added fish yet.
Is it OK/beneficial to push my drop checker past dark green to get plant growth going well?
My water is rock hard BTW.


1 Oct 2016
Yes as some if of the plants you will have will have been grown emerged ie out of water so they will be use too 400ppm CO2, the higher the [CO2] in the tank helps them make the change to submerged growth easier and can limit melting.

Plants need to adjust to this changing [CO2] which cost them energy so a higher [CO2] alone will not always help unless it is stable

we know that when the plant senses that high concentrations of CO2 is available, it responds by reducing the production of expensive Rubisco. When it senses a lower CO2 concentration it must increase Rubisco production, however because this protein is so complicated and heavy, the increased production requires 2-3 weeks in order to change the density in the leaf to match the new gas concentration level. So it is much easier to reduce production than it is to increase production. When increasing gas injection therefore, it hardly takes any time to see an improvement in health. When lowering the concentration, the plant will suffer because it must now ramp up Rubisco production to account for the loss of CO2 availability.

Hence the reason a pH profile is advised at to aim for max pH drop for lights on and stable for the main part of the photo period

What I mean is that if you are struggling with CO2 stability, or are having difficulty keeping high concentrations, then it's best to focus efforts on the first half of the photoperiod and not worry so much about the second half. If you are running very strong lighting then you are really pushing the plants and it's necessary to have the CO2 going. After 4 hours, if you turn the gas off, the water stays saturated for a couple of hours after the valve closes and the concentration trails off. So if you have an 8 hour photoperiod and you turn the gas on 2 hours before lights on, you can turn the gas off after 6 hours. So the gas is still running for 8 hours but it's on-off cycle is offset by the amount of time you turn it on prior to lights on.

And last but most important is flow needs to be good esp if using high lighting, diffusion in water is 10,000 times slower in water than air so water movement around your plants needs to be good to keep that CO2 high at the plants leaves, keep the light levels low whilst the plants make there adjustments and max photoperiod should be 5-6hours

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