You have a few things to consider:
Water movement - more movement helps.
Surface agitation - rolling surface without an oil slick provides oxygen exchange.
Water quality - running carbon actively (through a phosban reactor) keeps it more pure
Water type - tap water vs RO water will make a small difference as well.
Ambient air - if your house is sealed up tight for the summer, the CO2 levels will elevate in your home, which depletes the oxygen level in the tank. This lowers pH.
Sealed canopy - if the canopy is closed, fresh air isn't allowed into the tank, which lowers pH.
On top of that, what is your alkalinity, calcium, and magnesium levels? If you keep those are the recommended levels, the pH tends to stay where it belongs without buffering all the time.
Just so you know, if alk, ca, and mg are good, you really don't have to worry about pH at all. They are all inter-related. Many nice tanks are perfectly fine at pH 7.9 - (ref: Scott Michael's book Ultimate Marine Aquariums). Remember, stability is key. Keep the salinity and temperature stable (as well as your ca, alk, & mg levels), and your nano will do quite well.
