ph problem

My tank is a 55 gallon with 55 gallons of rock. I check my ph everyday was 8.1 about a week ago but it's slowly been going down now it's sitting at 7.8 to 8.0 I have some searchem marine buffer 8.3. Was thinking about adding it so the ph goes up. All I have is 2 blue chromis 2 peppermint shrimp and snails. Any ideas what it could be? Or if I can add this stuff so I raise the ph cause I'm worried for my fish. ALloyd I use is a protein skimmer hang on the back and 2 powerheads and no sump. What should I do? All levels read fine. I don't have any corals so I don't have any cal. Mag tests.