questions about co2

[wilkyb]

I've recently purchased a small co2 kit that I've been using for the past 3 days. I'd appreciate some your response regarding the observations I'll describe here:

I've noticed that very rapidly does co2 change the pH of water. After about 5 minutes of running the co2 diffuser in my 20 gallon planted farm tank has seen the pH go from ~7.8 to now ~6.8.

This leads me to believe that co2 simply ionizes the hydrogen particles to a controllable degree in the water very immediately after it begins running. About an hour after shutting down the co2 system then does the pH begin it's rapid ascent back to 7.8

What level of pH swing due to co2 injection is reasonable to expect more plant growth? At what point is it dangerous to the life inside of the aquarium? Am I on the right track here?

cheers!

Edited February 8, 2021 by wilkyb

[darkG]

I think it is useful to view the lowering of the pH and the better growth as separate effects. For your tank, at this time, there will be a correlation, but it will not be generally applicable. 

I don't know what pH is too low or the proper amount of CO² 🙂

[Daniel]

1 hour ago, wilkyb said:

This leads me to believe that co2 simply ionizes the hydrogen particles to a controllable degree in the water very immediately after it begins running. About an hour after shutting down the co2 system then does the pH begin it's rapid ascent back to 7.8

Ionization of hydrogen is one way to think about it, but a more productive way is to keep your eye on the carbon. That is why your are adding CO₂ right?

It is a back an forth equilibrium CO₂ + H₂O ⇌ H₂CO₃, then H₂CO₃ can lose one or both of its hydrogen ions yielding H₂CO₃ ⇌ HCO₃^- + H⁺ …. HCO₃^- ⇌ CO₃^2- + H⁺

When you add plants to your aquarium, they rapidly consume the carbon in the CO₂  during photosynthesis and give off oxygen. Here is what that looks like pH-wise in one of my planted aquariums:

The pH is lower first thing in the morning as the plants have been taking in oxygen all night and giving off CO₂, but as soon as the lights go on and photosynthesis starts, this reverses and the plants start taking in CO2 and giving off oxygen.

A normal pH swing of greater than 1 happens every day. I have seen daily swings greater than 2 with absolutely no distress at all from the fish. 

If you add CO₂ to a planted tank during the day time, your swings should be even less. Therefore even less cause for concern.🙂