"No girl is eating her fish."
Translation:Kein Mädchen isst seinen Fisch.
The same thing happened to me. If I remember correctly, Fisch is masculine; and in this case would be accusative? If that's true, ihr would have to take on the accusative masculine -en ending. Just like der Fisch turns into den Fisch, ihr Fisch turns into ihren Fisch. I hope this helps. If I was wrong or someone can explain it better, please correct me.