I'm confused whether goldfish will never stop growing or not, how much will its genetics affect its size?
I want to buy a high quality goldfish, not a bad one.
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Koi maximum about 4 ft , some have been documented to be about 100 years old ( in Japan).But I have seen some older koi limited to about 2 ft ; so genetics must be a factor . Koi and goldfish are closely related. In my small pond ( roughly 800 gallons ) the biggest koi has been about 32". Their size is greatly limited by the size of the water body . So if one has a 55 gal. aquarium , my guess is 12" would be about the maximum size goldfish. You need not worry about a goldfish becoming "too big". This is anecdotal but I have seen at least 50 ponds, public and private with maximum dimensions well over 100 ft.