The size of your goldfish's environment, along with other factors, will affect growth. It's a myth, however, that goldfish grow only as large as their environment. Most goldfish will die or have stunted growth if they do not have enough space in which to grow to their maximum size.
Myth
How big your goldfish will grow is determined by the animal's genetics, not by the size of his environment. If your goldfish doesn't have enough space to grow to his full size, he will likely die before you notice that the environment is not large enough. Even if your fish does survive in an environment that is too small, other problems are likely. These can include stunted growth, deformities and problems with scales and skin.
Growth
In addition to providing adequate space, there are other factors you control that will help your goldfish to reach his maximum size and have a long, healthy life. The cleanliness of your pet's environment, the kinds of food, and your feeding protocols are all factors. Genetics are the predetermined factor in how big your fish will grow. Goldfish sold as "tank suitable" will grow to about 10 inches while those sold as "pond suitable" will reach a maximum of about 18 inches.
Size
The size of your tank should be based on the size the fish will be when he is completely grown. The length of your tank should be at least seven times that of the adult fish's size. This means that if you have a "tank suitable" goldfish, your tank should be about 70 inches long. The height of the tank should be double the height of your fish. If your goldfish doesn't have enough room to move around, he can develop stress-related illnesses. Most "tank suitable" goldfish need a minimum of 15 gallons of water.
Fishbowl
A fishbowl is no longer considered an acceptable environment for goldfish. Bowls cannot provide enough space for a goldfish to grow to his adult size and cannot be set up to provide appropriate filtering and aeration. Most goldfish never reach their adult size in a bowl environment, even if the bowl is large enough. Bowls cannot provide the necessary setup for a fish to get required amounts of oxygen.
