Fish person

Fish people were humans who had been surgically altered by Damon of Atlantis to breathe water.

Biology
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. (TV: The Underwater Menace)

History
The Atlanteans ate plankton and used fish people to gather it.

Polly Wright was about to be converted into a fish person by Damon when she was saved at the last moment by Ara.

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 fish people regularly invited River Song and the Doctor to their annual parties. On one occasion in which they accepted, they were dipped in guacamole and tied upside down over a fire pit. After this River dubbed their invitations a trap and always sent them a decline.

Sometime later, upon receiving another invitation, the Eleventh Doctor wanted to attend one of the parties against River's advice. Once again, they were dunked in guacamole and tied upside down over a fire pit while the Doctor mused about how fish people could know so much about guacamole. (COMIC: An Adventure in Brine and Plaice)

A fish person was at Florana when Amy, Rory, and the Eleventh Doctor went there for a summer holiday. (COMIC: Summer Wholiday)

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)