It will drop like that when the water source your getting your tap water is over oxygenated. Does you water come from a surface source of from a well? Putting your water that you use for WC in a Barrell, toss in an air stone and see if it doesn't drop after 12 hrs.
The other reason this might happen is because not enough water is being changed and it's not changed often enough. This puts too much bioload in the tank so the pH will drop. What is your water change routine?
A constant drop will be detrimental to the fish over time. Discus require stability in their water parameters.