They are both correct, but have different meanings. The first one refers to fish as type of meat — a mass noun. The second one refers to individual examples of fish (the animal, not the meat), possibly alive, probably intact. In current usage, we never say “much” without an adverb before it, so we would really say “there is a lot of fish” or “there is lots of fish”. In your second example, “many” is used, although it might be more typical to say “there are a lot of fish” or “there are lots of fish”. I wrote this discussion for learners like you: https://www.italki.com/discussion/205633

There are many fish

Thank you

There are many fish. For example, there are many fish in the sea.