For a any amount of net CO2 loss, there is must be equal amount of non-CO2 gases replacing it. And a lot of it is O2.
I am not sure this is so. I do not think that it is a case of either/or. Somewhere I read that the gases in the water were independent of each other.
Must do more research here.
So before you seal the chamber, you have a steady amount of O2 coming into the system (and being absorbed into the water, otherwise it wouldn't be sustainable) and after you seal the chamber, you no longer have it.
YES! When a Wet/Dry is exposed to the air then the regular balance of elements in the air are reaching equilibrium with the water. As the air above the Wet/Dry circulates in the cabinet, in the room, and so on it matches the atmospheric levels for N2, O2, CO2 and other gases:
CO2 leaves the water because it is more abundant in the water (you are adding more of it via pressurized than the normal equilibrium would permit)
N may or may not enter or leave as a gas, but probably a bit leaves. Our tanks do have some small anaerobic pockets, and nitrogen gas is created here, in small amounts. It may just bubble out of the tank, or it may be raising the nitrogen gas content in the water, and leave via the Wet/Dry.
O is another anomalous element. Plants produce it. Fish use it. Plants use it when they respire. Is one or the other happening more than the other? If the fish and microorganisms are removing more oxygen than the plants are producing, then the water will gain oxygen when it is in contact with the air. If the plants are producing more than the animals are using, then oxygen will leave the system.
So by sealing your Wet/Dry chamber, you basically turn your Wet/Dry into another canister filter and you give up the one thing that a Wet/Dry filter does best: oxygenating the water.
Yes, you are indeed turning your Wet/Dry into a canister with a big air bubble.
The contents of that air bubble are going to be different than the atmosphere. One thing that will rise in that bubble of air is the CO2. Since there is a lot of CO2 in the water, it will start to leave the water when the Wet/Dry is first sealed, then stabilize when a new equilibrium is reached.
QUESTION: If you keep your CO2 running all night (when plants are not using it) will the equilibrium between the tank water and Wet/Dry bubble mean that even more CO2 will enter that bubble? I think so. Then, as the light comes on, will the Wet/Dry bubble donate some of that CO2 back to the water as the plants remove it? I think so. Like any equation: Add or remove something and the equation shifts to keep things in balance.
Now... how does that apply to the oxygen? If, overall, there is a net loss of gaseous oxygen in the system... then there needs to be a good re-supply mechanism.
If the fish, plants and microorganisms are removing the oxygen from the system and incorporating it into their bodies, locking it up, then there needs to be more and more oxygen added to the system.
Well, yes, that is happening. Whether it is part of the energy production, fish burn carbohydrates with oxygen; plants do a similar process; Microorganisms too, or incorporated into the body, oxygen is used for growth.
Are you running a Wet/Dry because of the enhanced O2 levels?
Nitrifying bacteria use a LOT of oxygen. If you were running a tank with no plants you would want all the N-cycle bacteria you can get. A Wet/Dry is great at this: By keeping the poret, bio-noodles or whatever exposed to the air, and making a very large surface area of the water by sheeting it over the bio media then the bacteria will grow like crazy in the highly oxygenated water, and the O2 levels in the tank stay higher.
But we are running planted tanks. The plants are a major N removal system. There must be a balance between livestock and plants where this increased oxygen level (to help the bacteria) is not needed, because the plants are doing more N removal.
Are those oxygen levels adversely affected by sealing the Wet/Dry?
I think the key word is 'adversely'. If we do not need so many N bacteria, then the oxygen need in the tank is reduced. The fish still need oxygen, but there might be enough as long as the water surface in the tank is gently rippling. The additional oxygen from the original Wet/Dry system is not so important when you are keeping less N bacteria.
However, if the reduced oxygen level from sealing the Wet/Dry is adversely affecting the system, is this so bad that you will NOT seal it, and put up with the CO2 loss instead?