The two main causes of low ph are 1.) low alkalinity or 2.) elevated CO2 levels.
1.) The alk test will tell you if this is the problem.

2.) Aerate some tank water outdoors for an hour and then check the ph. This will tell you if you have elevated CO2 levels. If your ph increases then you have to figure out why you have elevated CO2 levels. Some typical causes are:

a.) CO2 from Ca reactor. Fix: Turn down CO2 or add a 2nd effluent chamber.
b.) Insufficient aeration in your tank. Fix: Increase circulation. Macro algaes.
c.) Elevated CO2 levels in the room. Fix: Open the windows.