Substitute "biological activity" for "plants" and it all means the same. pH is highest just before the lights go off and lowest shortly after they come on.
I'll have to agree with the pH aspect. 7.2 is really low, even for a reading taken just after the lights have come on. Granted... pH buffers only afford a temporary increase in pH, but that doesn't mean that a low pH is unfixable, and should just be accepted.
I've got an article bookmarked at home about low ph causes/solutions and I'll post a link later unless someone beats me to it.
I won't get into the debate of whether nitrites are harmful or not, but I will say that in a properly cycled tank with a bacterial population large enough to support your bioload, you shouldn't see ANY nitrites show up.