Fish Farms were added to the Shipwrecked DLC more than a year after its official completion in the Home Sea Home update, after being tested in a beta branch for several months.
The three fishes that can only be obtained from Fish Farms are required to cook the Tropical Bouillabaisse.
According to the game files, it seems there was originally going to be 5th type of fish that could be grown in the farm, the "Spotted Wanda".
Gallery
All Fish Farm types. From left to right top to bottom, Tropical Fish, Newly stocked (Unset type), Unstocked, Purple Grouper, Neon Quattro, Pierrot Fish.