https://www.novelupdates.com/series/the-disabled-tyrants-pet-palm-fish-transmigration/

Description [Edit]

One day, Li Yu transmigrated into a fish. Furthermore, this fish even had a master. This master was the sinister and terrifying, mute tyrant from a novel. The system gave Li Yu a task. If he wanted to change back into a human, he had to obtain the tyrant's heart.

Li Yu full of tears: System, please wake up. I'm only a fish. He can't even speak. How can we interact?

System: Stop jabbering. To take down the tyrant, you must attack his heart.

Li Yu: Can I ask, does the tyrant have a heart–

Li Yu did his best to blow bubbles at the tyrant, but ended up being captured by the tyrant and raised carefully in a huge, ginormous...... fish tank. The life of a fish was filled with all sorts of blessings.

Everyday, Mu Tianchi watched the little carp swim around happily, pondering when this fish would finally transform into a human.

The little carp didn't know. It's not that the tyrant had no heart, it's just that he already gave it away to the little carp that accidentally saved his life.