Diagnose a goldfish's problem by examining its symptoms and possible causes. Some symptoms, such as sluggishness, can point towards multiple conditions. Goldfish can suffer from a number of diseases, such as anchor worm, dropsy and ulcers.
Poor water quality is a common cause of goldfish health issues. Living in a tank without a filtration system is deadly to a goldfish, as it may become ill and die when the water becomes depleted of oxygen. When a goldfish gasps at the surface for air, the tank has insufficient oxygen. Replace up to 40 percent of the water in the tank to alleviate the problem. For best results, a goldfish tank should be at least 20 gallons, and come equipped with a filtration system.
Refrain from using a goldfish bowl instead of a proper tank for a goldfish, as there is not enough room for the fish or a filtration system. Goldfish bowls also require frequent water changes, which cause stress to the fish, making it more susceptible to disease. A fish can also suffer from nitrate or ammonia poisoning from its own unclean water.
Other causes of goldfish stress and illness include improper temperatures, overfeeding or underfeeding the fish, too much algae in the fish's aquarium and constipation.