the airstone will only cancel out injected CO2. excel is a dissolved carbon and should not be cancelled out by oxygenation. however, there is not really any need for an air stone, so you could take it out. i know that some people who inject CO2 have their bubbles on an opposite timer as the lights, so when the lights go off the air comes on, and vice versa (because plants don't absorb CO2 while the lights are off, and CO2 can sometimes build up and suffocate the fish)