if i understand correctly a ph of 7 is way low comming out of the tap. tap ph should be at least 7.5. that tells me most buffering capacity is gone. plants, wood, unvaccumed gravel, any kind of acid that is in the tank would drop the ph low, quickly. i buffer my water with baking soda (cheap to buy). usually 1/2 TSP per 5 gallons works to keep the ph up around 7.8 until the next water change.
