Reasons why a goldfish is gulping air include poor water quality, overcrowding of the tank and oxygen deprivation. Goldfish require consistent cleaning and filtration of the water in order to keep the fish healthy. If too many fish are in the same tank, it can lead to bad water quality and not enough oxygen for the fish, causing the goldfish to gulp air at the surface of the tank.
Goldfish produce ammonia through their gills and waste, causing toxins to build in the water. When a filtration system is not used to remove or dilute the waste, it can lead to the fish being poisoned. Overcrowding — that is, keeping more goldfish in a tank than is recommended — lowers the available amount of oxygen in the water for the fish and increases the potential of poisoning by toxins.
To keep goldfish healthy, it is recommended that the first goldfish have at least 20 gallons of water. An additional 10 gallons of water is needed for each fish added to the tank. Owners who see a goldfish suffering from poor water quality can help alleviate the symptoms by cleaning the tank more frequently, changing the water and installing a proper filtration system for the size of the tank.