During the day when CO2 is on plants in your tank release oxygen. You also have surface tension and water circulation with the surface that will allow more oxygen in and will dissipate some of the CO2 being made. The warmer the tank is the less oxygen it holds.

At night people typically turn off the CO2 if they can. If not they have bubblers turn on to help with more aeration, but what it really is doing is gassing off more of the CO2. The reason this is important is that at night your plants reverse. Now they too are taking in oxygen and are giving off CO2.

Keep in mind that with the addition of CO2 you will see shifts in your pH. You may see daily changes of 0.5 to 1. If it changes more than that you are probably adding too much CO2 or need more aeration/surface movement.

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