Like Dan and Steve said, soft water will mess with an electronic pH meter and a pinch of sea salt will rectify the situation.
It sounds like you are working with almost pure water which is a great start for discus but I think that your crushed coral is doing a lot more than you think! It might have been preventing you from a catastrophic crash without you knowing it.
I would start from the beginning and use/get a chemical pH test kit to verify the electronic one:
Add a pinch of sea salt to your tap water (test)
Add a pinch of sea salt to your aged tap water (test)
Add a pinch of sea salt to your tank water (test)
The same test Al suggested but with an electrolyte (sea salt) for the electronic pH meter to work with.
If I were in this situation I would get a boat load of arm and hammer bicarbonate soda and make a saturated solution in a 5 gallon bucket (full bucket but with solid still present on the bottom) then start adding the same amount with every water change. Gradually increase it until you get yourself into a place with a reasonable margin of error/safety (Stendker have a KH of 8). Everyone loves doing water changes but sometimes life gets in the way and it'll only be made worse by coming home to a tank full of dead discus because you had a pH crash.