Matsya avatar came at the end of caksusa manavantara as discussed in this answer.
I see it mentioned online by many that Matsya avatar came in the satya yuga. However, I am yet to see any puranic or vedic evidence in support of it. Can someone share a vedic/puranic evidence or any Acharya's commentary to support that Matsya avatar appeared in the Satya Yuga?
After a Manvantara ends, there is Manvatara-sandhya. One may say that pralaya happened in the Manvantara sandhya. In the manvantara-sandhya, there are no yugas. We know from Matsya Purana 2.14, that King Satyavrata was meditating for 100 million years, then he saw Matsya avatara. Since it is described King Satyavrata was meditating, he has to be doing that during the Manvantara. After the Manvantara ends, there is certainly a pralaya and there is no space for meditation.
Since Matsya avatar came 100 million years after the rule of Satyavrata, then another way to ask the above question is when was King satyavrata ruling the Earth. Since the world was peaceful and he was engaged in meditation and meditation is the yuga dharma for Satya yuga, it seems that yuga was Satya yuga. However, we are looking for a direct statement from the Vedas or Puranas or Puranic commentaries.