"Fish" is a word used for a bad poker player. Usually someone, who is either completely new to the game, or they are just playing for fun and dont spend time reading poker books or watching training videos or anything like that. So you basically look for players, who do anything, which is not a good strategy:
* Limping into pots except from SB in games with antes
* Using bet sizes that are way to small or way to large
* Chasing after bad draws
* Overvaluing hands like top pair bad kicker
* Making ill adviced bluffs
* Showing down bad hands, that should have been folded preflop
* Donk betting on flops, that favour the opponent
In cash games you can add:
* Posting instead of waiting for the blinds
* Playing with a broken stack like 62BB instead of the full 100BB
* Buying in for an odd amount like 7,42$
For people using a HUD the main things to look for is:
* High VPIP (>40% for ring games)
* Large gap between VPIP and PFR
This is a statement, which is as old as poker itself, and its always made by people, who are serious about the game but also new and inexperienced. Or to say it in the less politically correct way: by bad regs. It has led to cliches like "maybe I should move up, where they respect my raises". But the harsh reality is, that if you cant beat the worst players in the game, then you have no chance of beating good players in more expensive games.
And its largely an emotional problem. So if I am to give only one advice, then it is to understand and truly accept, that the best player dont always win in poker, and that this is ok. In fact its not only ok, its the very reason, why its possible to make money. So its ok if the fish get lucky on you and river his gutshot or a random two pair like T2. This old video on the topic by CC member Evan Jarvis is still very relevant today: