The Bible is not a science textbook or an almanac. It's a book that describes who God is, what he has done and what he wants us to do through revealed knowledge, narrative history, folk tales and poetry.
https://answersingenesis.org/the-flood/how-did-fish-survive-noahs-flood/
“How did freshwater fish survive in the saltwater oceans?”
There are two possibilities. First, there are many areas in the world today where we see freshwater and salt water together, and the two waters don’t mix. So it’s possible that certain organisms survived in pockets of fresh or salt water. Second, because of natural selection, which creationists accept, organisms today have become very specialized. Organisms at the time of the flood, however, would’ve been much stronger and able to tolerate many more changes than they can today. There’s really no problem at all in answering this question.
The fountains of the deep are located beneath the surface of the earth. Researchers note there is sufficient water there to cover the highest mountains. I’ll get the link when I’m off my phone.
Fish were not brought on board per the text.