I would trust the API kits reading. If you only added a tiny bit of coral, I wouldn't imagine it shooting up the pH that high. For me, it's also too much of a coincidence for it to be the exact same value as straight from the tap. I'd blame a faulty pH reading from your previous kit. If that is indeed the case, the CC won't be necessary. However, once it is removed, keep it on standby and closely monitor the tanks pH to insure it doesn't begin to experience any drastic declines. If so, start adding a TINY bit in once again.
I also agree that pH should never be altered in 99% of cases...but a pH level of 5.5 falls into that 1% where it is necessary.
If fish are still dying off, it may also be a good idea to take a sample to the lfs to double check that the ammo, no2 and no3 levels you are getting is also accurate.
