The common Goldfish is a very miss-understood and often neglected fish due to its hardy nature and popularity amongst first-time fish keepers. Unfortunately there are many myths and miss-truths surrounding the acceptable care of Goldfish in captivity. Two of these are outlined in the text below.
In the aquarium the Goldfish is a hardy and adaptable species but can be a little boisterous, will scavenge amongst the substrate, and may eat or uproot delicate plants. Having said this, providing the aquarium is of a reasonable size, the Goldfish is an excellent display fish and does very well in aquaria. For ponds the Goldfish is ideally suited, being robust and brightly coloured, and this is the best environment for them. In nature Goldfish are opportunistic feeders and scavengers and will eat a wide variety of food sources. In captivity dried foods will provide an adequate nutritional diet but this should be supplemented with a variety of live or frozen foods. Although there are always exceptions, Common Goldfish will usually out-compete fancy varieties for food, and can cause damage through a boisterous nature so fancy and common Goldfish should not be kept together.
If you are looking for a first fish, or an 'easy' fish to look after, you should look elsewhere. The Goldfish can only be kept in large, coldwater aquariums and require exactly the same basic care and respect as any other fish.

The two great Goldfish myths:
Firstly, keeping these or any other fish, in unfiltered bowls is cruel and unacceptable. Many people counter this argument by stating that the fish will live for many years in such an environment. However, the reason why Goldfish are so popular and have been kept in captivity for thousands of years is because they are one of the few fish that are less likely to die under such conditions. This does not mean the fish is happy, just that it will tolerate high levels of mistreatment and cruelty for much longer than other species. Keeping these fish in unfiltered bowls exposes them to toxic waste products from themselves and their food which causes severe health problems and burning of the gill tissues. Without adequate filtration bowls or small aquaria are often exposed to large water changes in order to keep the water clear. These will stress the fish through temperature shocks and chlorine poisoning amongst other difficulties. Few people realise that fish are as capable of feeling pain, discomfort, and stress as any other animal, in which case keeping goldfish in such a way can be likened to keeping a cat or dog in a small box with its own waste matter and extremes of heat and cold for its entire life.
The second biggest Goldfish myth is that of its size, although there is a little truth in both arguments. In nature Goldfish can grow up to almost 60cm (2 feet), whilst in average sized ponds they usually grow to 20-30cm (8-12in), and in aquaria of a correct size they will usually reach up to 20cm (8in) The myth here is that the size of environment does not matter because the fish will only grow to a size which suits its environment. It is true that Goldfish will not grow large in a small environment; this is due to a growth gene that switches off when the fish is kept in a confined space. However, if the fish are kept in a very small environment they may appear to stop growing but will be prone to growth related deformities and early death caused by organ growth problems. Therefore it must still be considered cruel to keep the fish in small aquaria. At up to 20cm, and regardless of size because the fish needs swimming space, you will need at least a 120cm (four foot) aquarium to keep these fish properly. If you do not have an aquarium of this size, or other suitable housing, purchase something else instead.