Hello

I'm using a Profilux4 unit in a 540L freshwater aquarium. pH probe is used to control CO2 injection during the time lamps are switched on (I call it "day" here for short).
CO2 valve is completely disabled when lights are off ("night"), and controlled by pH probe when lights are on.

System works with no issue after calibration but two weeks later deviation in pH reading is so high that the CO2 valve remains closed most of the time.
I have replaced the pH probe, same behaviour, that is as follows:

- Calibration with 4.01 and 7.01 fluids => everything OK
pH value in Profilux matches pH value read by a Hanna HI98127, and also matches the aprox. value read by a Sera ph test (chemical test)
Values in aquarium:
at night (lights off, aeration on): 7.2
at day (lights on, aeration off, CO2 controlled by pH): desired pH value (6.5), after several hours of non-stop CO2 injection

- After two weeks:
pH value in Profilux does not match value read by Hanna HI98127. The reading of the Hanna HI98127 seems consistent with chemical pH tests and also with the CO2 concentration in water and Alcalinity.
pH value read by Profilux does not seem consistent with the measured CO2 concentration and Alkalinity.
Values in aquarium:
at night: Hanna reads 7.2, Profilux reads 6.85
at day: Profilux reads the desired value (6.5) but injection only last for two hours maximum. Hanna reads 6.7.

The deviation is not sudden. A small error can be yet seen first day after calibration. Deviation increase with the time.

Seems that the readings from the Profilux pH probe are more acidic (lower) than the reality, so CO2 injection is stopped before reaching the desired concentration.

Is this normal?
Should I be using a different probe, for freshwater aquarium?
Is this a calibration-related issue or is it caused more likely by biofilm in the probe? I clean the probe each time I recalibrate.

Regards

Pablo Gil