At equilibrium, the concentration of CO2 in the water will not equal the concentration of CO2 in the air. Instead, at equilibrium, their chemical potentials will be equal. At room temperature, CO2 likes being in a gaseous state more than in a aqueous state. Consequently, when their chemical potentials are equal, the concentration of CO2 in the water will be quite a bit less than that in the air.
According to https://climate.nasa.gov/news/2915/the-atmosphere-getting-a-handle-on-carbon-dioxide/ , the current concentration of CO2 in the air is 412 ppm. Extrapolating from this table (https://sites.chem.colostate.edu/diverdi/all_courses/CRC%20reference%20data/solubility%20of%20carbon%20dioxide%20in%20water.pdf) yields the following equilibrium concentrations of CO2 in water:
At 20 C: 29 ppm
At 25 C: 26 ppm
Extensive aeration, with room air, will help to maintain these equilibrium concentrations (you might want to consider a bubble wand in addition to air stones). Surface agitation would also help, but I would think the increase in surface area you would get with extensive aeration would be greater than that you would get with surface agitation. Plus, with surface agitation, you might end up with a concentration gradient of CO2. By contrast, aeration will help ensure the aquarium is equilibrated with the air at all depths.
Note also that the 412 ppm value is for outdoor air. Indoor air can be twice that or more, especially in a poorly-ventilated room with people.
The concentration of CO2 in water is approximately linearly proportional to its concentration in air, so if the indoor air were ~800 ppm, then the corresponding equilibrium concentrations in the water would be about:
At 20 C: 60 ppm
At 25 C: 50 ppm