Join Date: Oct 2006
Location: Ranchi, Jharkhand, India
The levels of O2 and CO2 are independent of each other and maintain an equilibrium position in your tank having a fixed relationship as the atmospheric air. One will not drive out the other.
Most plants and fish are happy as long as the O2 and CO2 are at this equilibrium position. There are some fishes who like the water to have a little more O2, for example the hill-stream loaches. There are some plants which like the CO2 level to be higher like your baby-tears.
This equilibrium position does change; at night, or lights off period, the CO2 levels increase and oxygen levels decrease due to respiration of plants and fishes. During the lighted hours the opposite happens, due to photosynthetic activity of plants oxygen is given off and its levels increase and CO2 is absorbed by the plants so that its level falls. The diffusion of gasses through the water surface tries to equalise and offset the imbalanced equilibrium position, but this process of correction is low as the tank has a very limited surface area. Consequently a slight imbalanced position exists.
In planted aquariums, especially in well-lit tanks, most hobbyist add CO2 in one form or another to drive up the photosynthesis by plants and consequently the growth of plants, so that they over-compete with algae and keep them down. Most keep the CO2 level a little higher than the equilibrium position.
This higher CO2 level does have the effect of reducing the ph of the tank water. This drop in ph is in direct relation to amount of CO2 in excess of the equilibrium position. This drop in ph is due to the formation of H2CO3, a weak acid, too much of which will make your fishes gasp for air, even when the oxygen level is higher than equilibrium in the tank water.
The trick is to keep an eye on drop in ph of your tank, a drop checker (it checks the drop in ph level hence the name) and several electronic ph controllers are devices that helps you do that; although the most reliable method is to observe the fishes and and adjust the CO2 supply to a level they show no sign of distress. What you do is gradually, very slowly over days, increase the rate of CO2 supply (do that only when you have the time to observe and monitor the activity of the fish) and at the first sign of distress roll back the increase and fix it there.
If you have a choice, you have a problem, till you elect your choice. No choice, no problem, only consequences, learn to live with them.