It's 'are' because it is plural, 'Fish' is a confusing noun, because in most cases the plural form is the same as the singular. It's the same as 'sheep' - one sheep, two sheep. So you can say: There is a fish in the pond. There are some fish in the pond. The verb form tells you that the first is singular and the second is plural, even though the word 'fish' doesn't change.

Fish can be either singular or plural. There is no change in the plural form of fish.

It is either - one fish or a fish is singular. A sheep or deer likewise, but maybe not cattle.