The problem wasn't that fish had learned how to fly. It was that sharks had adapted to follow them.

Mystic Skyfish

Whenever you draw your second card each turn, Mystic Skyfish gains flying until end of turn.

Foundations Jumpstart

Jumpstart 2022

Alchemy Horizons: Baldur's Gate

Core Set 2021

If a spell or ability causes you to put a card into your hand without specifically using the word “draw,” it's not a card drawn.

The triggered ability can trigger only once each turn. It doesn't matter Mystic Skyfish was on the battlefield when the first card was drawn. If it's not on the battlefield when the second card is drawn, the ability can't trigger at all that turn. It won't trigger when the third or fourth card is drawn.