To eliminate the possibility of tap water causing fish dying, I would suggest to get a TDS meter and see how bad your tap water is. I still can't believe that the corals and inverts are healthy if water is suspected to be contaminated. They are supposed to be more sensitive to water quality than fish.
HM Digital Handheld TDS Meter w/ Thermometer
If you had pwc and then all the fish died then yes I would suspect the tap water. Like you said you have been using same water for 9 months. It would take days and you would have noticed fish to be stressed out before they died and not just overnight. I still strongly believe you have MV.