24/7 CO2 is the problem lah, I am also running 24/7 CO2 as my regulator very hard to get precise bubble count... But do you not have the same problem using the previous light setup? Funny, light shouldn't have an effect... Infact should improve O2 saturation during Lighted hours.
So I still suspect during light-out hours with CO2! Fishes, plants, bacteria, etc... takes in O2 and CO2 is given off in addition CO2 supply! thus O2 level drop naturally.
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