Aquarium pH keeps dropping, why?

I don't know anything about fish aside from bettas; however, I do know that your water could possibly be the culprit. Some water (city or well) fresh from the tap comes out at one PH and after a while starts either going up or down. The solution for those people was in letting the water sit out that they would need for water changes. I believe they normally left it out for about 24 hours.