That pH isn't an emergency, and may not be a problem at all. What is your bioload? What time of the day did you measure the pH? What kind of water do you use to top-off? What is your pH usually? Has anything died, or disappeared, lately? How often, and how much, do you change water? These are the typical causes of lowered pH, although you haven't convinced me that it is really low at this point. Repeat the measurement several times during the day.