This is a bit of a trick question, really, with a few different answers. If you want to get super technical (not to mention outrageous) about it, carp – the breed goldfish are a relatively small species of – could theoretically grow to be 600 pounds in size. In fact, in Thailand’s Chao Phraya River, there are already some believed to be that big alive and kicking (or swimming). The largest carp ever actually caught, however, only weighed 94 pounds – still no small feat, but a big difference from some of the still mythical South Asian variety.
Either way, those fish are quite different from the little orange swimmers we have in our at home aquariums. When it comes to the standard goldfish, the size goes way down. While an old wives’ tale claims that a goldfish can grow as big as its container, the largest on record was caught in 2010 by a schoolboy in Dorset, England. That one weighed in at an adorable five pounds and measured 16 inches long. What an achievement!
But about that old wives’ tale – how big will a pet goldfish REALLY get? If you put it in a swimming pool, could it grow that big? Well, yes and no. Tropical Fish Magazine explains it thusly:
There is an element of truth to this, but it is not as innocent as it sounds and is related more to water quality than tank size. When properly cared for, goldfish will not stop growing. Most fishes are in fact what are known as indeterminate growers. This means that, unlike humans, they grow until they die. What really stunts a fish’s growth is poor water quality and improper care. In smaller aquariums or bowls, water quality is typically very poor. With little or no filtration and infrequent water changes, goldfish suffer. The stunting that results is not a good thing. Rather, it is a sign of ill health, and, frequently, stunted fish take on a deformed appearance and die at a young age. As already stated, some goldfish grow very large, so it is important to know what size your fish will attain before purchase and to make sure you can provide it with the proper care it requires.
And there you have it! That might make you think again before you purchase a goldfish for yourself or your kids thinking it’s a simple, small pet. You could end up with more than you bargained for!