Calibrate the pH probe. Or if it's two years old and has not been maintained just get a new one. pH probes are not "put it in the tank and forget about it" type items.
Unlikely... as the water from the tap is 100ppm so to add that to the tank and then the tank drop to under 10ppm is not likely.If it's a short-term result, IE, you just did a water change and the KH & pH fall immediately, that's puzzling and would probably warrant some investigation, particularly if that water change didn't stress or kill your fish. Your current values aren't too bad for the fish you've got..
I suspect this is the route to take - was kinda hoping others had experience with a KH crash - I suspect the Mulm in the flourite bed but not positive.If it's a result of not having done a water change in a while, then I think the best you can do is to do small daily water changes (together with gravel vacuum) of 10% or so until you gradually get to your target...
(course I never mentioned and should have, that I normally kept the tank at pH of 6.6 to 6.8 via CO2 injection. The CO2 has not kicked in for quite some time now)I would worry about the 7.8pH and the discus, though.