[essabee]

Hoppycalif is right and so is Amano.

Andy T got the right point. It is the baseline.

Hoppycalif's baseline is the ph at 30ppm CO2. Amano's baseline is at perhaps that at the light on point.

It is certainly a fact that if the ph is higher 4/5 hours after the light on point, the CO2 supply is not equal to consumption.

Also those who have aerator on at light off point will have to find their own baseline.

I think it is totally subjective what you want to do with your CO2. Personally I have decided not to go for maximum growth, only sustained growth, that means fiddling with supply so that the ph goes slightly up or stays level at the end of the light on period.