I am still interested regarding O2. Probably i am overthinking it, but if the water is to the top, say 1/2"-3/4" to the glass, and the glass covers the tank very well, with little room at the edges. Does that impact how much O2 is dissolved actually in water from the current rips of the filter? Can that create O2 deficiency for the fish, while CO2 is run at 25-30ppm? How that squares?
Your question regarding O2 is really important and you are not overthinking it !!
When you have a tightly covered tank with little room at the edges, you essentially have a CLOSED system! Under these conditions Henry's law has to be applied when it comes to estimating the amount of dissolved gasses in the tank.
states: "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid."]
This means, if you sparge a single gas (in your case CO2) into a CLOSED Tank system, this gas will eventually replace all other gases in that system. In other words, given enough time CO2 will drive O2 out of your tank (not just oxygen, but nitrogen too). Consequently, your fish will suffocate due to the lack of O2. [NOT due to the presence of CO2]
However, in the planted
tank things are somewhat different. As long as the light is on, the plants photosynthesize and produce ample of O2 in situ
, so your fish have enough O2 to breathe (min. of 4 ppm is needed).
I want to emphasize that the danger of driving O2 out of your tank ONLY exists for the CLOSED system. If your tank is OPEN, the gasses (over the water column) mix so fast with the surrounding air that you can no longer drive O2 out of your tank i.e. below critical levels.
THIS IS THE MOST IMPORTANT DIFFERENCE between an OPEN and glass COVERED (CLOSED) aquarium!