"Magnificent, isn't he? So peaceful, and so content."
– Dejaren, 2374 ("Revulsion")

Spectrum was the name of a holographic fish that the HD25 Isomorphic Projection Dejaren created to keep him company. The Serosians didn't allow Dejaren to have the fish as a pet, so he was forced to hide Spectrum from them.

In 2374, he introduced The Doctor to Spectrum. The Doctor found him to be rather lifelike. (VOY: "Revulsion")

Spectrum was played by a porcupinefish (genus Diodon).