As far as aquariums go,monster fish are those that cannot turn around easily in a two foot wide tank.
With my current tank a monsterfish would be anything up to 6inches
IMHO (very humble), I think you've got to take into consideration size, appearance and behaviour.
Take a 3ft African tiger fish... It's a monster, truth be told, it's practically a koi with teeth...but no matter how big a koi you show me, it ain't no monster.
It's not the size of the fish but the size of fight in the fish.
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