The halo fish was a type of sensitive lab-animal kept in sickbay on board the USS Enterprise. This species of fish, which was normally surrounded by a bright glowing halo of light, lost all color at the smallest environmental change.

In 2269, Captain Kirk called sickbay to see if the spiroid epsilon waves the ship had been just probed with had any effect on the sensitive lab animals. Nurse Chapel reported that the halo fish were glowing as bright as ever.

Once the crew and all organic material aboard ship began to shrink, it was observed by Chapel that the halo fish had shrunk to tadpole size. (TAS: "The Terratin Incident")