How would you make "fish" plural possessive?
The possessive plural of fish is either fish's or fishes'.
Fish is a noun which has two plurals. "I caught five fish". "The fishes in the river include trout and salmon". Whether the plural is fish or fishes depends on context.
If the plural form of "fish" is "fish," the possessive plural is "fish's". If the plural form is "fishes," the possessive plural is "fishes'."
Examples of the possessive plurals are: "These are my fish's children" and "My fishes' names are Blenny and Eel."