1. Tim likes fish, but Susan doesn't like it.

Question

2. Tim likes fish, but Susan doesn't like them.

Which pronoun do I have to use?
I think 'fish' is used as a plural noun?

Answer 1

Yes, "fish" can be either singular or plural, depending on how it's used in a sentence.

I'd use the first sentence you wrote; in fact, I'd probably say, "Tim likes fish, but Susan doesn't."

=)