Tell your Dad that what he is doing is like playing Russian roulette with his Oscar.
Simply placing the minnows in to a quarantine tank, and observing them for disease and parasites for a few days/weeks is also far from fool proof. Wild fish can harbour any number of pathogens, most which cannot be seen with the naked eye, pathogens that a wild fish may have built up a natural immunity against. Introduce that same pathogen to a tank raised fish, that has no natural immunity for that pathogen, and the end result is typically a very sick (or possibly dead) fish.
Some might suggest to medicate the feeders, but to medicate indiscriminately can also cause its own set of problems. One needs to know exactly what they are treating for, or you risk strengthening the immunity of other pathogens & creating super bugs by using improper dosages. And who wants to be feeding mega medicated soaked feeders to their fish on a constant basis?
If your dad wants to feed live feeders, the safest & most responsible way is to raise his own. There are plenty of low cost fish that will produce tons of fry on a regular basis.
