how long has the tank been setup?
stocking?
water changes and frequency?
ph and kh of your tap water might also be nice.....
i'm sort of thinking your tap water is not that low to begin with and the nitrification (?) process produces acids to lower it, which could be compounded by overstocking...... i would imagine water changes will bring it back up some, and even if the kh is low, a slight amount of crushed coral in the filter might be enough to stabilise it somewhat.....
good luck
edit: forgot to ask tank size too.......