Maybe because they can't express their feelings like other animals do.

I think he is saying that fish don't have human consciousness or emotions.

ini lagu nirvana kan?

Thank you sir for your explanation :)

Curt Cobain's lyrics did not always have a literal meaning. Most songs were just mixed thoughts and rambling with a loose theme tying the words together. The particular line of that song would likely have been rooted in thought that it was okay to eat fish during lent on Friday when other meats were off-limits (Fasting in Christianity).