The main thing, fish do wrong, is to play to many
hands. Other fish tells include:
* limping preflop
* posting
* weird stack size (not buying in full, not topping up)
* donk betting a flop, that favour the preflop raiser
* weird bet sizes like a min-bet or a massive overbet out of nowhere
The points about posting and weird stack sizes only apply to cash games. Also limping can be a reasonable strategy in some tournament situations, when stacks are short. Finally it is obviously also relevant to observe, what hands people show down. If he has something, he clearly should not have played, then you dont need a large sample to conclude, he is a fish.