Have you happened to test your tap's water pH?
pH is a measure of H+ vs OH-. The more H+ vs OH-, the lower the pH.
H+ are normally increasing in fish tanks due to the acidic processes going on, pretty much naturally in every one's tank. The pH remains the same if buffering to counteract the increasing H+ is available(KH counteracts by the means of OH+ and HCO3 - mainly) but if KH is not replenished on time, there'll be no more buffers to cope with the increasing H+ and the pH drops.
Hardness is generally a measure of calcium and magnesium ions and is not directly correlated with pH, although water high on those ions is more likely to have higher pH, but it is not always the case, and vice versa.
Having said that, fish die and many of the times we don't know why or our suspicions can be wrong. I wouldn't worry too much about it as long as you've ruled out water quality.