It's doubtful that it has anything to do with chaeto. If fact, lighting chaeto 24x7 should raise you ph. You need to provide a lot more detail. Tank size? Are you dosing? If so, what are you adding and how?
What test kits are using? Have you verified the numbers using another test kit? Are you running a CA reactor? What type of flow do you have? Etc.
If your tank chemistry is messed up and you don't understand why, the safest thing to do is stop dosing and do water changes.