Why do domesticated fish have a size dependent on their living space?

Question

Why do domesticated fish have a size dependent on their living space? I thought goldfish were supposed to be fairly small but they keep finding giant ones in Australian rivers. If one was to buy a big enough tank, could one theoretically convince a goldfish to grow to a much bigger size? Why does this size increase happen – is it purely food consumption?

Answer

Although considered a small, domesticated species, goldfish are, in reality, a colourful subspecies of carp, a wild freshwater fish that can grow to over a metre in length. They were domesticated in China around 2,000 years ago, and selectively bred for a variety of (mostly aesthetic) reasons. Although there are a wide variety of different breeds of goldfish, and some are naturally larger than others, the sad truth is that most pet goldfish do not get very big simply because they don’t live long enough.
The natural lifespan of goldfish can, in good conditions, exceed forty years. However, pet goldfish often only live for a few months or years. This is mostly due to them being kept in unsuitable conditions – goldfish have quite a high metabolism (for fish), so they require a habitat (artificial or otherwise) with a large area of water surface to allow oxygen to diffuse in. Fish in low oxygen conditions will often be seen gasping at the surface, as this is where the oxygen concentration is highest. If they cannot get sufficient oxygen, the fish will suffocate. Goldfish also require quite a large volume of water, relative to their size. This is also due to their high energy expenditure – as they eat quite a lot, they also excrete quite a lot. Fish excrete their waste as a chemical called ammonia, and the levels in the water will quickly build up and damage them if this is not dealt with by frequently changing their water, or using an efficient filter. Ammonia buildup also frequently kills pet goldfish.
Under conditions of both low oxygen and high ammonia, goldfish which survive are often severely stunted, meaning that they grow much slower than they otherwise would. This has led to the myth that goldfish’s size is intrinsically linked to the size of their tank – in fact, their growth is often severely restricted by poor conditions, or by premature death.

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