The low pH is probably caused, in your case, by an accumulation of co2
because of the glass tops. What is your current alk
This article should help.
Low pH: Causes and Cures by Randy Holmes-Farley - Reefkeeping.com
"Buffers alone are not generally a good method as they raise pH little, and result in excessive alkalinity. Unfortunately, the labels on many commercial buffers are written in ways that convince aquarists that the pH will be fine if they just add some buffer. More often than not, the pH is not improved for more than a day, and the alkalinity rises beyond desired limits.
Two other useful methods include growing macroalgae that absorb some CO2
from the water as they grow (often lit on a reverse light cycle to the main tank to provide the maximum pH rise when the main tank is at its pH minimum), and aerating the water with fresh air