It is a myth that a goldfish will grow only to the size of their tank. If a goldfish is kept in a small tank, fed only a small amount once daily, and the water changed regularly, it will grow only very slowly and may live for several years, reaching a maximum size of maybe 3 to 4 inches (8 to 10 centimeters). It is true that the growth of a goldfish may be somewhat stunted in a small tank – a high nitrate level is known to stunt growth – but growth is slowed, not stopped altogether.
However, if fed too much, or if kept in a tank that is not sufficiently filtered or maintained, a goldfish will simply grow to the point where its tank will not support its wastes and will succumb either to poisoning as a result of waste build-up or to disease caused by poor water quality.
The main factors which control the growth rate of goldfish are feeding and temperature.
Goldfish will grow faster if they are fed a higher protein food, or are fed more often, and, given an adequate food supply, they will grow faster the warmer the water temperature. In ponds, goldfish usually grow quite slowly, as their growth rate is minimal over winter.
They may grow to around 6 to 8 inches (15 to 20 cm) but possibly more. If kept indoors in large, or heated, aquaria they will reach this size sooner and potentially grow even larger. Straight-tailed varieties will attain a greater length than twin-tailed types, but since twin-tails are fatter, their actual mass may be even more.
Like the maximum size, the lifespan of goldfish is also variable. The record is 43 years, but it is uncommon for goldfish to live this long. Goldfish usually live quite long when kept in large aquaria or outdoor ponds, up to 15 to 20 years is not unheard of. In smaller or heated tank, a lifespan of five to ten years is quite achievable. As mentioned, the most important factors when keeping goldfish in small tanks are careful feeding and adequate filtration and/or maintenance.