If you have kept a single goldfish in a fishbowl, you probably know that it doesn't live for a very long time. You probably have also seen many goldfish in a single, small tank at the local pet shop, but that doesn't mean it's the most healthy environment for the fish. A 45 litre (10 gallon) tank might seem big enough to accommodate more than one goldfish, but in realty, one goldfish per 45 litre (10 gallons) of water is the most healthy environment for the fish.
The gallon per fish rule is debunked
Goldfish, whether they be fantails, fancies or koi, are meant to live outdoors in a pond. They can live past 30 years of age and can grow up to 30 cm (1 foot) long. The common belief that a fish will only grow as big as its environment will let it is simply not true. A goldfish kept in a fishbowl will not be restricted in growth, it will merely die way before its time.
Goldfish are messy
Goldfish expel a lot of waste. Placing too many goldfish in a tank will result in dirty, cloudy water and could lead to disease or ammonia/nitrate poisoning that will wipe out all the fish in the tank. Likewise, the single goldfish placed in a goldfish bowl will suffer the same fate.
Too many is bad
Goldfish require a lot of room to breathe -- literally. Too many fish in one tank will deplete the oxygen supply and cause the goldfish to stress and eventually die. Although you can put oxygen into the tank by means of a filter or air hose, goldfish need surface air, and each one requires roughly 0.28 square metres (1 square foot) of surface area per 2.5 cm (inch) of its size.
One goldfish in a 45 litre (10 gallon) tank might seem like one lonely fish. Goldfish are very friendly toward one another and will even eat from a human hand once they've become accustomed to their caregiver. So if you want to keep more than one goldfish, you will need a bigger tank or an outside natural or man-made pond.
Aquariums and pet shops aren't in the business of keeping goldfish as pets, so you may very well see many goldfish in one tank. When buying a goldfish, there are several things to look for. Make sure the tank water is clean and the fish are swimming around in a healthy manner. They shouldn't be lethargic or breathing too fast; both are signs that the water is low on oxygen, causing the fish to be stressed. Make sure the fins, tail and body are in good condition. Goldfish are covered with a protective slime coat -- when it is "eaten" away, the fish is prone to infections.