"the nitrogen cycle" will drop it naturally. acids and softer water are by-products of the cycle......
but: depending on how it's dropping, where you started and how fast it drops might be an indicator of problems. overstocking and poor maintenance or not enuf water changes can cause this to happen pretty rapidly.
what's the original ph of your tap water and how fast does it drop? how often do you do water changes?
baking soda will work, but usually only for a short time. some crushed coral or shells will also help too.
but i do agree i would be looking into why it's happening.......