CO2 doesn't cause algae outbreaks.
People who say they have 27 ppm, for example, of CO2 are fooling themselves. If they are using the KH/pH/ppmCO2 table, and they probably are, they very likely have a lot closer to 5 ppm than 27. Even the drop checker, which removes the tank water, with all of its various pH changing substances in it, from the equation, can only tell you that you have somewhere between about 25 and 45 ppm of CO2 when it is green. If you have any trouble distinguishing the shades of green apart, you won't even get that accuracy. And, that reading only applies at the position in the tank where you measured it, at the time that you measured it, so it is just a tool to persuade you to increase the bubble rate enough to get close to the optimum amount of CO2 in the water.