The goldfish is hardy, but give him a simple fish bowl for a habitat and you'll stunt his life span, because the water quality will suffer. A goldfish can stand such an existence for a year or two at best; but to reach his potential, he needs a filtered environment that can process the large amount of waste a goldfish produces.
A goldfish can survive for a short time in an unfiltered small bowl or in a tank with poor water quality, but if he lives in unfit water for very long, his life will be shortened considerably. Healthy goldfish live on average 15 to 20 years, while those kept in unhealthy conditions, even well-cared-for otherwise, live an average two to three years. A filter is necessary to maintain the type of water quality a goldfish needs to live a long and healthy life. Using a filter doesn't mean you can't keep your goldfish in a bowl -- as long as it's a large one. Filters are available for traditional tanks, decorative containers and goldfish bowls. The smaller your fish's habitat the greater the need for filtration.
The filter uses a pump to draw water out of the aquarium and pull it through a system typically composed of mesh, aquarium floss and activated charcoal, although the exact materials vary between types of filters. The filter cleanses the water of pollutants that harm goldfish, including excrement, plant and food detritus, chlorine, nitrates and heavy metals. It encourages beneficial bacteria to consume organic waste by trapping it in the filter medium. Without a filter, these contaminants remain in the water and reduce the amount of oxygen available to your goldfish, making him prone to bacterial and fungal infections.
Two basic types of filters are available: in-tank and exterior. Both are suitable for goldfish. In-tank filters that submerge work best for bowls and small tanks that have no room to hang a filter on the exterior. Exterior filters attach to the outside of a tank or inside a cabinet and pull water out of the tank, cycle it through the filtering mediums and return it to the tank.
The filter should be able to cycle the water volume of the tank 10 times an hour. For example, a 10-gallon goldfish tank needs a filter that will cycle at least 100 gallons every hour, regardless of filter design. Also, even with a filter installed, it is necessary to perform weekly water changes to maintain cleanliness and pH balance. For tanks or bowls that hold less than 10 gallons, change about 25 percent of the water by removing and replacing it. For tanks larger than 10 gallons, change about 15 percent of the water each week.
If adding a filter is simply not an option, you must perform water changes frequently -- at least twice a week or any time the water starts looking cloudy or dirty. Your goldfish gulping for air at the surface of the water is an indication that the water is so dirty it has been depleted of oxygen. Keep the tank or bowl away from sunlight, which encourages algae and bacteria growth, and consider adding live plants to absorb carbon dioxide and add oxygen to the water.