Small aquarium fish that lose their glowing colour at the slightest change in their environment, kept aboard the Enterprise and used as a living indicator in The Terratin Incident.
Halo fish are small ornamental fish surrounded by a soft glow. They are extremely sensitive to their surroundings and lose their colour when conditions around them change even slightly, which makes them useful as a natural warning signal.
They appear in the animated episode The Terratin Incident, set in 2269. A tank of halo fish is kept aboard the Enterprise, and when the ship is bathed in the strange radiation from Terratin the fish turn colourless, an early sign that something is affecting all the organic matter on board.
The episode pairs them with the gossamer mouse as the two laboratory animals observed while the crew shrinks.