Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Lets think this though, and you can compare the conditions to your tank.
If you are not adding CO2 from pressurized or yeast, then the other sources are all the sources there are.
Fish: If the tank has a LOT of fish, this is a pretty good supply of CO2. It is also pretty well distributed in the tank, even without a lot of water movement. Keep the fish happy, you need enough water movement to keep the oxygen level up, but no more water movement than that. Gentle ripples at the surface seem the best.
Decomposing material: Anything with carbohydrates has carbon, and this can be released as CO2 by the decomposer organisms (bacteria, fungi). Too much agitation of the water will allow this CO2 to reach the air, and leave the tank. But it is a slow source, though pretty constant.
Equilibrium with the air: This is a very low source. Gently ripples at the tank surface and good circulation so the CO2 rich water is circulated throughout the tank allows the maximum amount of CO2 from the air to enter the tank.
All the gases in the air can enter the water. But if the water is still those gases do not move very fast within the water. We want high levels of oxygen for the fish, and high levels of CO2 for the plants. But if the water is not moving the surface water will be in equilibrium with the air, but the water lower down will be whatever the fish and plants make it. Usually this is low CO2 (The plants are using it as fast as the fish are creating it) and low O2 (The fish are using it as fast as the plants can make it). So for the sake of both plants and fish, it is better to circulate the tank water.
However, too much agitation of the surface seems to drive off more of the CO2 than is gained by good circulation. Gentle ripples at the surface seem to be the optimum.