The big question is WHY is it 5. You can add baking soda, but your water will just keep becoming depleted of buffering if you have something like extremely high nitrates. Is there anything in your water that could be making it acidic? Have you checked and cleaned your filters, and siphoned the substrate? Could you have a dead fish or something else decaying in the tank? Do you test your nitrates?
Does your tapwater test low too?
You should test the kH both in tank and in your tapwater. If your tapwater reads low you can add baking soda until you attain an adequate kH reading, and the pH will come up with it. You will still want to run crushed coral in your tank (which is best or maintaining pH, but will not significantly raise it in all conditions), and you'll need to test your tankwater kH periodically to see if it's being maintained. If you don't test you may not know your buffering is depleted until your pH drops significantly again.
And of course, last but not least, are you SURE your test is reading accurately?
