Co2 won't go away that easily. What plants do you have? The best solution IMO is get an air machine and blow some bubbles into the tank for a day or two until the checker goes back to normal.
Yeah try that. And how many bps do you run when the co2 is on?
Ok when the co2 is on, try dropping the bps to 1bps. Sometimes the plants can't take that much co2 and a lot of it just goes to waste.