If I have not known and eaten by mistake non-kosher fish. Please advise what to do? What prayers to say or more?

The 4 steps of Teshuva are as follows: a. Leaving the mistake that you made, to stop eating that fish. b. Sincerely accepting upon yourself in the future not to revert or repeat eating that fish, and resolve to try learning as much Judaism as you can, so as not to make similar mistakes. c. Sincere regret of the past mistake. d. Saying the "Viduy" as printed in the siddurim (in nusach Sfarad before tachanun every day, or at the following website with transliteration and English translation- I don't necessarily hold from this English translation, but this is what I found readily available). Don’t recite or relate to the first paragraph on that site (which is just for someone, chalila, on their death-bed) but just recite the 2nd 2 paragraphs beginning: Eloheynu… and Oshamnu… with their translation: https://www.chabad.org/library/article_cdo/aid/364287/jewish/The-Viduy-Confession-Prayers.htm It's extremely important not to feel depressed about the mistake, which in fact, was inadvertent and accidental, and not let it disturb your serving Hashem with joy and happiness!