Sorry Hopeful, can't help you here.
It's a mystery to me - I can't even begin to give a possible explanation or diagnosis from the info at hand, and based on your fishs' behavior. This is one for the experts - could be any number of things - your Dad's fish took a year before all died - first time around - that says something -- many negative things can occur during that period of time.
Where are you located ? Is there any possibility your well water & your Dad's municipal water could be from the same originating source ?
On the other hand - it may not have anything to do with the water at all.
Regrets - this is not my area of expertise.
Paul