A German word that this German can't do anything with. It seems to be very dialectical. :´)
Mal sehen, jo lässt sich eigentlich immer als "doch" übersetzen. In diesem Fall ist es, je nach Betonung, also entweder "Ist der Fisch weg? - Nein... Na gut, er ist fortgeschwommen. - Der Fisch ist also doch weg." oder "Warum hast du den Fisch nicht gefüttert? - Der Fisch ist doch weg. - Ach richtig." Ob das jetzt deinem "fei" entspricht, musst du sebst wissen. :´)
I think "jo" rather corresponds to German "ja". According to ordnet.dk "jo" is 1) used to express astonishment and also 2) if you say something which is already known by those you are talking to. This is exactly identical to German.
Fisken er jo væk. Meaning 1: Der Fisch ist ja weg. (One moment ago, it was still there) or meaning 2: Der Fisch ist ja weg. (Though you already know it, I mentioned it once more)
Is "obviously" a good translation of "jo"? I'm not sure, but if it has a similar meaning, then that might be better than "I tell you". Saying "the fish is obviously gone" makes sense in English, but it also matches the Danish word order, so know that "jo" translates to an English adverb like "obviously" would also help learners understand where it should go in a sentence because it would be like English.
Like I said, I'm not sure if this is correct. I'm not a native Danish speaker, but this is my understanding of how the word is used, so please correct me if I'm wrong.