I've been doing more reading and research and would like to say I was wrong about a dwarf angel being ok in a 20 long.
It's not a matter of gallons per say but the environment a person can create in a space that size. By the time you have your lr
in and scaped hiding spots for an angel, the living room is gone.
Will a 4" angel fit. Yes.
Will said fish live as healthy life as "humanely" possible? No
It turns into a goldfish situation. They can live 15+ years in a pond or BIG tank but throw them a fish bowl and you've taken that life potential away.
Sorry for my previous uninformed suggestion.