"The freshwater fish does not drink water!"
Translation:Peștele de apă dulce nu bea apă!
"proaspătă" because "apă" is feminine. But you should always be aware of idioms and fixed phrases, "apă dulce" (lit. sweet water) is the Romanian way of saying "freshwater".
I think Românian is not the only language with uses "sweet" instead of "fresh". And why would you call it an idiom, as the fresh water isn't literally fresh either, it's only less salty
We Dutch also say :"zoet water". The salty water we call "zeewater" (water from the sea). The water in between sweet and salty, we call "brak water". Fresh water (right from the tap or tge spring) we call "vers water". But this is not a course "Dutch language", si I stop now, hahaha...