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

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?

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."

=)

In this case, you would actually use the pronoun "it" because "fish" is being referred to as a singular noun.

In sentence 1, "fish" is singular and is used as a general term for the food that Tim likes. Since "fish" is singular, we use the pronoun "it" to refer to it.

In sentence 2, however, "them" is used because "fish" is plural in this context. If you were referring to a specific number of fish, you would still use "them" (e.g., Tim caught three fish, but Susan doesn't like them).

So, in summary, when referring to fish as a general concept or a singular noun, use "it." When referring to specific individual fish or using fish as a plural noun, use "them."