Do fish alter the Ph in aquariums?

I'm having problems with my fish dying so I tested my water and noticed my PH is VERY low. So I decieded to test my tap water and its damn near perfect. The levels are where I would want it on everything. But my question is, do just having fish in there affect the PH? It's just a peaceful little community 20 gallon tank. All of my gravel and ornaments were purchased from PetSmart so there is nothing that I've just "found" and I do have live plants in there, also purchased from PetSmart. The plants wouldn't cause problems like that right? I'm just very lost >.<