Śrīla Jayadeva Gosvāmī sings:

pralayo payodhi-jale dhṛtavān asi vedaṁ vihita-vahitra-caritram akhedam keśava dhṛta-mīna-śarīra jaya jagad-īśa hare

Soon after the cosmic creation, the entire universe was inundated with water. At that time Lord Kṛṣṇa (Keśava) incarnated as a gigantic fish to protect the Vedas. Therefore Manu addresses Lord Matsya as mukhyatama, the first incarnation to appear. Fish are generally considered a mixture of the modes of ignorance and passion, but we must understand that every incarnation of the Supreme Personality of Godhead is completely transcendental. There is never any deterioration of the Supreme Lord’s original transcendental quality. Therefore the word sattvāya is used here, meaning pure goodness on the transcendental platform. There are many incarnations of the Supreme Lord: Varāha mūrti (the boar form), Kūrma mūrti (the tortoise form), Hayagrīva mūrti (the form of a horse) and so on. Yet we should never think any of Them material. They are always situated on the platform of śuddha-sattva, pure transcendence.