"Airstones", to me, means a sandstone-like material that is made to get a column of air bubbles in an aquarium. I tried one on the inside of a CO2 reactor, similar to the standard "Rex Grigg" reactor, with the CO2 going through the stone. After about a year the stone was just a pile of sand, completely disintegrated. I know that isn't what you asked, but I have been reluctant to use them in CO2 enriched water since then. I know Plantbrain has recommended using one at night, with air, to more quickly drop the CO2 concentration in the water. That, to me, means using an Airstone and CO2 both operating at the same time is a waste of CO2.