It's actually very useful. You have to consider the adult size of the fish when using it though. It's not 1 gallon for every 1 inch fish. It's 1 gallon per inch of fish.
It's not a hard and fast rule though. For example, you're not going to put a 5" cichlid in a 5 gallon tank and expect it to survive very long. It does work for stocking purposes in example cardinal tetras are about an inch or so give or take as adults so I could put 5 in 5 gallons. With proper maintenance and filtration, they'd most likely flourish.
Was he ever a fisherman? Catfish might interest him. I'd also suggest simple tetras, maybe colorful glowlights (if dad can see color), or black neons. While betas are interesting, if [when?] it jumped out and if it died, dad would feel bad.
The opportunity to watch something alive and lively (that's not TV) may perk your dad up.![]()
I'd go with a Dwarf Puffer and an oto... The puffer should be very intereactive with him and the oto would be good for any algae that would grow.