Halo fish

From Memory Alpha, the free Star Trek reference

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

In 2269, Captain Kirk called sickbay to see if the X-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")