Yep, it's the key. Otherwise you sit in a loop of bacteria consuming nitrate then dying and then PO consuming bacteria growing and consuming PO then dying and so on and so on. My guess is your PO is low because your pellets are consuming it instead of nitrate. Once you remove the PO the nitrate bacteria will take over.
The ph thing is simple enough to explain, as the bacteria grow and feed heavily they remove oxygen and increase carbon dioxide. Once they hit the balance point the levels all return and the ph goes back up. That's how you know things are balanced and you can add more.
I never measure my PO remover. I just add some and replace it monthly or when it stops removing PO.
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