Berachos 40a

Gemara: "Rabbi Chiya bar Ashi said in Rav's name: One who regularly eats small fish will not get intestinal diseases. Additionally, small fish make one's whole body fruitful, multiplicitous and healthy."
Contrast with the gemara on 44b: "The Rabbis learned: Small, salted fish, kills at times..." (Look there for the full context.)