I am Spanish (I am learning English, sorry if there are any mistakes) and we use pez for the animal (hay un pez en el río/there is a fish in the river) and pescado is for cooked fish (when it is already cooked) or to fished fish that it is going yo be sold in a supermarket and after that we are going to cook it in our houses
because „de„ is not„ with„. in English, you can mix the meaning / translation of the prepositions as you like (eg, to, in, with, etc.) making them synonymous. In logic, prepositions have clear and strict rules on prepositions, having one, very rarely) three translations, compared to English, where you can find at least 8 translations for each preposition.