I was also going to suggest O2 deprivation, so instead I will second your last theory. Chromis seem to be very susceptible to O2 deprivation & be among the first to die. Tangs also have high O2 requirements. If CO2 levels are too high, this for sure will drop O2 levels since water, especially saltwater, can only hold so much dissolved gases.
Sorry for your losses.
Anthony
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