I would even expect Pinguine fressen Fische, as it emphasises that they eat the animals rather than the meat. After all, you don't say that wolves eat venison. Please report it during the lesson.
Singular is more common here, like Löwen fressen Fleisch, Kühe fressen Gras, Pinguine fressen Fisch. But also "P. fressen Fische" is a valid translation of "P. eat fish".
Maybe penguins eat fisch with good table manners, that is why they do not FRESSEN
Juliogmz1939, I love your sarcasm! This is a monumental error of Duo and it has been reported many times with no action on their part. Tiere "fressen", Menschen "essen".
Duo should either explain or correct the fressen v essen issue. This comment has been here for a long time.
sure seems like it should be fressen, unless there's some obscure exception for penguins!