Eli5: What causes goldfish grow to such extreme sizes in the wild?
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Their relative longevity. Goldfish grow continuously for their entire lives. Aquarium goldfish usually succumb to disease or mishandling while they remain small. However, if you manage to not kill your goldfish, it will just keep growing!
That’s the size they normally would be, if they were not stunted by being kept in a small bowl. The goldfish you see in small tanks at shops or given away as prizes, are usually young fish raised in stressful conditions.
[https://injaf.org/the-think-tank/do-fish-grow-to-the-size-of-their-tank/](https://injaf.org/the-think-tank/do-fish-grow-to-the-size-of-their-tank/)
It is often believed that fish “grow to fit the size of their bowl”, but what is really happening is pathological stunting due to inadequate living conditions for their needs. That’s partly about space, but also cover to hide in, light, “privacy”, food, water quality, waste, etc.
Wild fish stunt when overpopulated and underfed, etc.
Your thinking is backwards. Those are not extreme sizes, those are the natural sizes for those animals. That is how large they are *supposed* to get: in the wild, *Cassius auratus* can live upwards of 20 years and grow to 24 inches or more. It is goldfish in captivity who are severely stunted, usually because of insufficient care, poor water quality, and small space.
To be absolutely, 100% clear it is a myth that fish “grow to the size of their tank.” The reality is that they are just dying early and young because they don’t have enough space, and the lack of space is causing them to be deformed and stunted. It is not healthy for them.
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