"The fish from the table is good."
Translation:Peștele de pe masă este bun.
Because they are interchangeable in almost every context. "Este" is just that little bit more formal.
I thought "the table" was masa. I guess it's an idiom, where in Romanian you don't use the definite article in that phrase?
You could take it as an idiom. A more detailed explanation is that you don't use the articulated version of "masă" here, because it's clear which table the speaker talks about.
You have the same phrasal feature in Italian: "da tavola"/"da tavolo". Perhaps it is a trait amongst Romance languages.