I do normally see an inverse graph but it is never as drastic as that. Only seen it since condensing my NOPOX dosing between 9am and 4pm. I am not seeing any adverse effects but thought it noteworthy to mention. I would like to find an alternative to NOPOX as I would like my pH higher. I am thinking of an algae reactor run at night but as I have a nano with no sump, I am trying to avoid more pumps and pipes in the main aquarium!ORP naturally falls as pH rises. Might that be the nature of the ORP change you are seeing?