A goldfish indicates stress by gasping at the surface of the tank, refusing to eat, swimming in strange patterns or developing white spots on its body known as "ich." A goldfish may also develop fungus or a bacterial infection, manifested as white cottony growths or red spots, respectively. Sluggish and abnormal behavior indicate that a fish is being subjected to consistent low-intensity stress.
Fish Channel recommends identifying and rectifying the source of the stress as soon as possible. Sudden changes in environment, bullies or uncomfortable tank conditions are the primary causes of fish stress. Although diseases like dropsy and fin rot can induce stress, it is more likely that stress has left a fish open to infection or parasites rather than the other way around. Easy ways to prevent stress are regular and frequent water changes and maintaining consistent temperatures and pH levels. Monitoring the behavior of all the goldfish to make sure none are bullying the others is also key; because a goldfish is confined to the aquarium, it cannot escape the territory of the aggressive fish, and the continuous stress can lead to death. Separating the offending fish from its tank mates can restore harmony to the aquarium.