From Latin PISCIS

ESSERE SANO COME UN PESCE - to be as healthy as a fish

NUOTARE COME UN PESCE - to swim like a fish, to be a perfect swimmer

NON ESSERE NE’ CARNE NE’ PESC - to be neither meat or fish, to be in between, to not have a specific flavor, personality, etc

ANDARE A FARE COMPAGNIA AI PESCI - to go keep company to the fish, to drown

NON SAPERE CHE PESCI PRENDERE - literally not knowing which fish to catch, to fish, to have ran out of options, therefore being uncertain about a the future 

‘Non riesco a trovare a trovare lavoro! Ho provato dappertutto! Non so più che pesci prendere’
 ‘I can’t find a job, I tried everywhere, I don’t know what to fish anymore!

ESSERE COME UN PESCE FUOR D’ACQUA, to feel like a fish outside the water, very uncomfortable

‘Mamma mia, ieri alla festa mi sono sentita un pesce fuor d’acqua! Erano tutti dottori e scienziati e non sapevo neanche di cosa parlassero!' 
‘Oh my God, last night at the party I felt like a fish out of the water(out of its element) they were all scientists and doctors and I didn’t even know what they were talking about’

TRATTARE QUALCUNO A PESCI FACCIA, to slap somebody with a fish, to treat them BADLY

‘Wow, sono andata in banca per chiedere un prestito, e mi hanno trattata a pesci in faccia’

‘Wow, I went to the bank to ask for a loan, and they treated me horribly’

CHI DORME NON PIGLIA PESCI, the one who sleeps doesn’t catch fish

ESSERE MUTO COME UN PESCE, to be as mute as a fish, won’t talk, won’t reveal a secret

BUTTARSI a PESCE, to throw yourself as a fish into something or into doing something 

‘Ha cominciato il nuovo lavoro ci si è buttato a pesce’ ‘He started a new job and thew himself in it, like a fish’

FARE L’OCCHIO A PESCE MORTO, to give the dead fish eye, which, despite what the image would suggest, refers to looking at somebody in a seductive// very sweet way

PESCE D’ APRILE, what English speakers call April fool

L’OSPITE È COME IL PESCE: DOPO 3 GIORNI PUZZA, A guest is like fish, on the third day it starts to smell — can’t stay more than 3 days as a guest!