Is fish uncountable? Im a bit confused why it is not "se on kala"
"Se on kala" means "it is a fish". The partitive case indicates that we are talking about some, not one.
For example when you ask what kind of dish something is, right? It's fish.
Yes. If there were an entire intact fish on the plate, then nominative case would also be an option.
you, sir, are a fish.