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What is typical CO2 ppm in a low tech tank?

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Internet says pretty much every number.  I've heard everything from 2-6ppm and one video a guy claimed to have 15ppm in his low tech no-co2-injection planted tank.


From the Henry's law constant for CO2 is 29 ATM/(mol/L) and there's ~400ppm CO2 in the atmosphere so

.0004 ATM /(29(L ATM/mol)) *44g/mol = 0.6mg/L = 0.6ppm

Do fish really breathe that much to have the co2 so far out of equilibrium?  I feel like I must be doing the math wrong here.  Can anyone clue me in here?

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Wow.  Thanks Irene!

In link 2 this post (#12) explains a lot.  Though it raises more questions to me that I'll have to look into. 

The TLDR of that post is that carbonate hardness (KH) is very important for dissolved CO2 in water and that's something I'd never heard before.  This may influence my algae issue too.

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