The pig's pen is in the field.
The pigs' pens are in the field.
The pig pens are in the field.
A fish's home is in the sea. <children>
The fishies' homes are in the sea. <children>
The fish's homes are in the sea.
The fishes' homes are in the sea.
From How Can I Correctly Use Apostrophes?
When a plural word is the same as a singular word, such as the word fish, apostrophes can get a bit more confusing. If you are discussing a single fish, the correct form is fish's to indicate possession. Words that are pluralized with es, generally indicate possession by placing the apostrophe at the end of the word. Some scientists use the term fishes to talk about more than one species of fish at the same time. For example: The difference in these fishes' habitats is amazing.From en.Wikipedia
Fish does have a regular plural form, but it differs in meaning from the unmarked plural; fishes refers to several species or other taxonomic types, while fish (plural) is used to describe multiple individual animals: one would say "the order of fishes," but "five fish in an aquarium." From Grammar Blog
Fish: the plural for one species of fish, or caught fish, is fish, but for live fish of many species, in poetic usage, and in some dialects fishes is used.
Fish's habitats or fishes' habitats would be correct because the plural of fish, ... You will never see something like fishs' because there is no such word.