What's the pH of your water coming out of the tap? What is in the tank and how long has it been set up? Sometimes rocks or driftwood can leach out bicarbonates or tannins that can lower pH. You might want to double check your results against the testing kit in your local fish store.
I'd recommend checking out this link about low pH in the aquarium. It is inconvenient at best and harmful at worst to continually have to adjust pH.
After checking for these things, it might be easier to know how to advise you how to proceed. HTH~