A little while ago I set up a pressurized CO2 system, consisting of a 5 pound CO2 tank, a Milwaukee regulator with bubble counter and solenoid, a ceramic disk diffuser, and a drop checker for good measure. For the first little while (over a couple of days) the drop checker stayed blue. I kept increasing the bubbles per second trying to nudge the dissolved CO2 upwards. I guess I nudged to much and my drop checker turned bright yellow. I turned down the CO2 and waited a couple of hours but the drop checker stayed yellow. I turned the CO2 off waited a couple of hours again and the liquid remained yellow. I performed a number of water changes and added an airstore and the drop checker remained yellow. I pulled the drop checker out of the aquarium and placed it on my desk, where it sat and remained yellow for 2 weeks until it finally turned blue again. At no point during this time did the fish show any sighs of stress that might be associated with excess CO2. So I'm curious if anyone has ever "blown out" (for lack of a better term) their CO2 drop checker? Has your liquid refused to return to the blue colour?