The Matsya avatara is usually considered as an avatara of Lord Vishnu.
But the Shatapata Brahmana and the Mahabharata both describe Matsya as an avatara of Prajapati Brahma. While the Puranas describe Matsya as an avatar of Lord Vishnu.
Do some scriptures, other than the Puranas, describe Matsya as an avatar of Lord Vishnu?
Yes , you are right . The earliest account of what was afterwards regarded as an incarnation of Vishnu is found in the "Satapatha Brāhmana." It will be noticed that though in this passage a wonderful fish is described, it is not said to have been an incarnation of any of the gods. The Mahābhārata says that Brahmā assumed this form; whilst the Purānas teach that the fish here spoken of was Vishnu.
But As described here Here
This transfer of work from one deity to another is not a matter of much surprise, when we remember how frequently it is declared that all the various gods are but forms of one supreme being.
This is clear from the below verses from Shreemad Bhagvat Purana.
श्रीभगवानुवाच अहं ब्रह्मा च शर्वश्च जगतः कारणं परम् ।
आत्मेश्वर उपद्रष्टा स्वयन्दृगविशेषणः ॥५०॥ahaḿ brahmā ca śarvaś ca jagataḥ kāraṇaḿ param
ātmeśvara upadraṣṭā svayan-dṛg aviśeṣaṇaḥLord Viṣṇu replied: Brahmā, Lord Śiva and I are the supreme cause of the material manifestation. I am the Supersoul, the self-sufficient witness. But impersonally there is no difference between Brahmā, Lord Śiva and Me.SB 4.7.50
आत्ममायां समाविश्य सोऽहं गुणमयीं द्विज ।
सृजन्रक्षन्हरन्विश्वं दध्रे संज्ञां क्रियोचिताम् ॥५१॥
ātma-māyāḿ samāviśya so 'haḿ guṇamayīḿ dvija
sṛjan rakṣan haran viśvaḿ dadhre saḿjñāḿ kriyocitāmThe Lord continued: My dear Dakṣa Dvija, I am the original Personality of Godhead, but in order to create, maintain and annihilate this cosmic manifestation, I act through My material energy, and according to the different grades of activity, My representations are differently named. SB 4.7.51
So considering above references and assuming that all the Tri-Murties are forms of one supreme being. It makes not much difference. Just Transferring of the credit , Just the name change.
No, the avatar of fish is of Lord vishnu-
Here is what sage narayana says about his avatars in Santi parva of mahabharata-
Appearing in the forms of a swan, a tortoise, a fish, O foremost of regenerate ones, I shall then display myself as a boar, then as a Man-lion (Nrisingha), then as a dwarf, then as Rama of Bhrigu's race, then as Rama, the son of Dasaratha, then as Krishna the scion of the Sattwata race, and lastly as Kalki. When the auditions in the Vedas disappeared from the world, I brought them back. The Vedas with the auditions in them, were re-created by me in the Krita age. They have once more disappeared or may only be partially heard here and there in the Puranas. Many of my best appearances also in the world have become events of the past. Having achieved the good of the worlds in those forms in which I appeared, they have re-entered into my own Prakriti.
Anyway- prajapati just means the "Lord of all beings" it's a title used by both Brahma and his sons who are first patriarchs of this world ex prajapati daksha.
Prajapati is foremostly the name of Lord Vishnu- as prajapati means Lord of all beings, similarly lord vishnu Vishnu is also prajapati not of all this world but of Brahma too.
Even in many prayers Lord Vishnu is called prajapati. It comes in Vishnu sahasranama, he is foremost of prajapatis.
So the prajapati matsya is avatar of Lord Vishnu.
Indologists with their half knowledge of sanskrit write many wrong things brahma and brahman is written very identical in Sanskrit language- so where matsya is mentioned with Brahma it was most likely the brahman-Vishnu not jiva Brahma.
