You really need a better method of diffusion than an airstone you need to hold the gas in the water column for a longer period than just bubbling through an airstone straight to the surface. You are probably wasting a lot of your Co2 using your existing method. You could still use the airstone method as long as you had some sort of contraption to keep it under the water for longer ie: bell jar. Upturned jar so as the bubbles from your airstone fill the jar which is below the water surface and when the jar becomes saturated it will release the excess. You will find a significant increase in the aquarium Co2 levels and reduction in pH if you adopt this method. Then you will have to regulate it.
Regards
BigC