I agree that tough fish usually means overcooked fish.
Sometimes people describe big/old fish(meat) as being tough when what they really mean is that it has a stronger flavor( gamey)?

Poaching in a seasoned liquid is a good way to handle toughness/gamey issues.
You don't want to overdo it though or you'll end up with fish/meat that resembles a rubber tire.
Poach until fork slides in with little resistance and call it good.