Ha'dagim sochim amok mitachat lamayim.
Sentence with Hebrew audio @ https://forum.duolingo.com/comment/17044424
הדג שוחה עמוק מתחת למים
That would be "the fish swims", not "the fish swim". The verb indicates whether fish is singular or plural.
You're right, I realized it seconds after sending my post but couldn't find it and delete it :D