Thomas Aquinas

Similarly, if a fish. The fish lives in water and signifies understanding of dogmas: "He that drinks of this water will not thirst again" (Jn 4:13); and in v. 14: "It will be a fountain of water springing up into life everlasting." Again, by water, tribulations; hence, by fish, those living in the waters of tribulation. Or fish refers to faith, which is hidden under the water, i.e., the guardianship of the spirit; but the false doctrine of heretics, by the serpent.

Commentary on Matthew ↗