Please help with tap water ph vs tank water ph. Need some advice :)

Because there could be gasses and solids in the water straight from the tap that will outgas/precipate out after 24 hours. In other words, the reading straight out of the tap might not be all that accurate- another possible factor between the difference in readings you're getting between the tap and tank.

If your driftwood and substrate are pulling the pH down that far after only 12 days, that effect most likely is only going to be temporary.

What kind of substrate are you using?

Also- are you injecting CO2? If you are, then you need to set some tank water up for 24 hours to let the CO2 outgas before testing.

Do you know your kH and gH readings? Those two are actually much more important parameters for livestock, anyways.

(PS- I realize I keep asking you more questions without answering your original question... but until we understand exactly what's driving the fairly dramatic pH shift that's occured in the 12 days since you filled this tank, I don't think we CAN accurately answer your question...)

I think you may have fallen into a trap that the hobby has created for us. There is a huge stream of info saying that PH is a critical item for fish health. It seems to be one of the main points we read about.
Buy then there are also many sings that this may not be true. Take as an example, African cichlids. Among the first things we read is that they have to have hard alkaline water to do well. But a number of this group find this is not the case for our fish. I find my is doing quite well and continue spawning when I have added CO2 and gradually driven the PH down from 7.8 to as low as 6.8. A full point in PH drop. In fact using a controller to read the PH so that I feel I am getting a reasonably accurate reading, I find the PH in a tank varies quite a lot from day to night when CO2 is not running.
So the question becomes whether PH is really that important item that we are all reading about?
I congratulate you on wanting to do the best for your fish but then I feel you may be somewhat mislead by advise you read. I would not follow PH as the guiding light that it may seem to be. I am finding that the water hardness may be a much more important factor.