Fish people were humans who had been surgically altered by Damon to breathe water.
Some of the fish people had scaly skin, large, fish-like eyes, gills and fins. Others had undergone only partial conversion and resembled humans with gills and goggles to protect their eyes.
The Atlanteans ate plankton and used fish people to gather it.
Sean and Jacko convinced the fish people to go on strike as part of the Second Doctor's rebellion against Professor Zaroff. (TV: The Underwater Menace)
The Twelfth Doctor invited Clara Oswald to meet "fish people", describing them as "fish. And people". She returned to her date with Danny Pink with seaweed in her hair. (TV: The Caretaker)