I am having a similar problem.
I have a 20 Gallon glass (tall) tank. I have gravel, 1 plastic plant, 1 ruin ornament, 2 black moores, and 2 calico fantails. I have an aquaclear 150 filter (b.o.t.), and a bubble wall for oxygen.
I have to change 20% water every week to bring my PH back to safe...