Sorry to hear your bad luck MrV. Hope you can recoup!
Perhaps some heavy metal in the water that conditioner did not remove? Metals tend to be additive over time and not an immedately noticable thing. But can be fatal. Perhaps have your water tested, and a good idea to switch to bottled water for drinking for a time being until you get results? Like the canary in the coal mine, these little creatures can detect lower levels of toxins than we can....
A good time to re-organize a new fish room!
Best of luck!