Spectrum was the name of a holographic Serosian 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. Dejaren was eventually destroyed when B'Elanna Torres ran a surge of raw power through his matrix, but the holographic emitters on the ship remained functional; it is unclear what happened to Spectrum after Dejaren's death. (VOY: "Revulsion")