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.