I think the term "fish" evolved from "fishing" for a hand hence technicality the player is really a fisherman , I would identify the "fisherman" in a table game as VPIP over 50%
Over time bad player become know as the "fish" as typically player would get bad beats from a player "fishing" for a hand and paying high pot
odds make a call and got mad and calls the player a "fish"
Typically post flop play a passive calling game without regard to strategy couple with high VPIP is a "fish" as well
As this term is so wide opinionated in definition I generally regard a "fish " as one that opening up way too many hand without regard to position preflop
How they fish play post flop is more player specific there are passive and aggressive fish
Sometime I see a regular who gets drunk or tilted and then starts playing like a "fish"
Sometimes the whole table recognizes when someone starts playing like a "fish" and trips over themselves and plays mediocre starting
hands just to get into a hand with the 'fish" and becomes the "fish " themselves
I think it better to get position (left of the fish) and let the fish come to you .
So what I am saying is that a "fish " to me is a extreme loose player preflop