The plants/fish/food and the driftwood both assist in the acidity you mention.
try taking the wood out . And it sounds like your not doing 10 to 20 percent water changes often enough to keep your tanks p.h. close to your local tap waters p.h.
go to wallmart , petco, petsmart or another local fish store and buy either jungle or mardel brands water testing strips. Get a 5 in 1 test an a ammonia test. Test your tank and tap water with 2 strips each to double check and your problem should clear up if you are doing enough water changes. What size tank, filter fish? How many fish, plants, snails? What are your other parameters NO3- NO2- NH4+ before and after water change? How often do you do water changes? How much water do you change at one time? What type of filter media, and how often do you change it? all of these things can affect p.h.