Guys, Problem is that the Drop Checker is now constantly showing yellow. I use the 4 dkh solution from AE which is normally changed every other water change (fortnightly) but this last change it went from blue to green after a few hours (as normal) and remained green for about 3 days but then slowly turned yellow. CO2 is switched off at night via solenoid. I've even turned the CO2 off during the day and it still remains yellow. I've changed the solution but the above process is repeated. My guess is the 4dkh solution is suspect and will order some more from AE. Has anyone else experienced this problem?