Before you try messing around with pH what are the other parameters, especially KH and Nitrate? Good idea to test the tank and tap water with all the tests you have, and then to repeat the pH test on the tap water that has sat out for 24-48 hours.
Carbonates are one of the most common buffers in aquariums. If the KH is dropping for some reason, the pH is highly likely to drop. KH can drop for any of several reasons, for example, water changes with water that is low in KH, and certain plants will utilize KH as a source of carbon, there are other things going on in aquariums as well that can lower the KH. Certain substrates remove the KH from the water, allowing the pH to drop.
The link between nitrate and pH is this: In a tank with few water changes and rising nitrates the KH and pH will get lower. It is part of a group of things that happen. Look up Old Tank Syndrome and see if this is going on.
If I told you that baking soda is the cure for the problem without helping you to see what else might be going on this would not be a proper answer.
IF you discover that the KH is dropping for some reason, and can figure out why you may decide that adding a source of carbonates can help. Crushed coral, oystershell grit sold for caged birds, limestone based sand or fine gravel are all possible sources of carbonates, as well as calcium and magnesium. (Have you checked the GH, too?) Baking soda is a source of carbonates without the other minerals, but does add sodium to the tank.
IF you discover that your tank is suffering from Old Tank Syndrome, suggested by rising nitrates, then adding baking soda will not solve the underlying problem. You need to remove nitrogen from the tank faster, and other organic wastes either by improving conditions so the plants can help you out, or by increasing the water changes (or both).
IF you find out that your tap water's pH drops after being exposed to the air overnight you may have to figure out some other solution, starting with talking with the water company about their water chemistry.