If you’ve been trawling (no pun intended) the depths of the news, underneath all the negativity you might have spotted the interesting story of the ‘world’s biggest goldfish’ caught by a British angler at a fishery in France. The aptly named ‘The Carrot’ topped the scales at a whopping 30.5kg – almost double the previous record-holder’s weight. The Carrot was subsequently returned to the lake to await the next lucky angler.
It was reported that The Carrot was placed in the fishery’s lake 20 years ago and has subsequently grown to gargantuan proportions. This got us thinking. How big can goldfish grow? Could a pet goldfish get this big? Should we release all these fish so they can grow massive?
Table of contents
What Are Goldfish?
Goldfish (Carassius auratus) are a type of carp that likely originated in China. It is believed that some carp were born with a mutation giving them orange and yellow scales, rather than normal silver. These fish were selectively bred together to create a new domesticated variety of carp. In Imperial China, gold was a symbol of wealth and royalty. So it is easy to see how the golden fish were attractive. From China, they spread around the world and became common worldwide. There are now over 300 breeds of Goldfish.
There is some debate about the taxonomy of goldfish, and some fish sold as ‘goldfish’ may not be Carassius auratus. In fact, The Carrot is reported to be a hybrid of a leather carp and a koi carp; meaning she is not considered a true goldfish. These two carp are closely related to C. auratus but cannot reproduce to produce fertile offspring. Bear this in mind, as some carp species can grow much larger than goldfish.
Do Goldfish Grow as Big as Their Tank?
This is a common myth often heard, particularly to encourage owners to have a goldfish in a more convenient small tank. We’ve actually written an article discussing why goldfish bowls are not considered humane anymore and one of the reasons why is the small size.
In many cases, goldfish do stop growing in relation to the size of their tank. Goldfish growth is driven by growth hormone. Somatostatin/growth regulatory hormone is a hormone that inhibits growth hormone release. It is theorised that in smaller tanks, the concentration of somatostatin will be higher. Thus suppressing growth of the fish, also known as stunting. Other factors, such as diet and health also cause stunting.
Unfortunately, there are few studies investigating stunting and whether it harms the fish. Some people argue that fish that are not allowed to grow to their maximum size will have muscle wastage. They also put forward that despite the external fish staying the same size, the internal organs continue to grow. This can lead to organ damage as the enlarged organs push against the growth-restricted body. Overcrowding in tanks would likely compound both issues.
When we look for evidence, the effects of stunting are still up in the air
A study on farmed milkfish, that were kept in conditions considered to cause stunting for varying lengths of time, found that all groups had a period of compensatory growth after their restricted periods to make up for their stunting. They even found that milkfish restricted for 8 months had higher protein and better-quality meat than non-restricted fish. This implies that stunting can be reversed and may even be beneficial in certain farmed fish. A similar study on diet-restricted goldfish fry showed similar results, with all of the larvae having compensatory growth once given an appropriate diet. A final study in goldfish with restricted space and feeding found that survival rates for fish at higher stocking densities was reduced, though feeding did not affect survival rates.
Despite a mixed bag of evidence, it stands to reason that intentionally restricting the size of a pet goldfish’s tank is inhumane for the various reasons we’ve mentioned in our previous article (though stunting may have benefits for commercial farmers). You should always strive to have an appropriately sized tank for your fish.
One textbook recommends up to 144cm2 of water surface area for every 2.5cm of fish length (not including their tail fins) – this is the water directly exposed to the air (another reason why narrow-necked goldfish bowls are not good for fish). This stocking density can be increased with air-producing stones, waterfalls, fountains, vents, plants and so on.
So How Big Can Goldfish Grow?
As the previous section explained, a goldfish’s size may be partly affected, for ill or not, by the size of their tank. So how big could a goldfish in a huge lake grow? Because there are other species of fish that are sometimes considered ‘goldfish’, it is difficult to answer that question. Goldfish tend to live 10 years of age if well cared for but can live for decades in ideal conditions. Since they do not stop growing, older goldfish can reach much larger sizes. Varying sources online state that most pet goldfish grow to around 15-25cm and weigh a few hundred grams.
However, there have been stories of larger than average goldfish found in tanks and ponds worldwide. The current Guinness World Record holder was 47cm long in 2003 and lived in The Netherlands. The BBC reported a goldfish bought in 1998 an inch long grew to 38cm and 0.9kg by 2008, despite being kept in a tank (perhaps more proof that not all fish remain stunted in tanks). Two goldfish left in ponds in England and the USA were 41cm and 2.3kg, and 38cm and 4.1kg respectively.
It’s not clear if all of these big fish are goldfish, or related species, but clearly when in the right conditions, whether that be a large tank or a pond, fish can grow quite large. Likely not many, even when cared for correctly, will reach these sizes, but with some luck yours might.
Should We Release Goldfish to Grow Large?
You might be wondering if it would be okay to let your goldfish swim free in a nearby lake or river, so they have a chance to grow big. And while this might happen, far more likely are some unintended negative consequences.
Most likely will be that your pet fish will not survive their new environment. Sudden changes in water quality, temperature, salinity and contents can cause rapid death to fish. This is why pet shops recommend slowly mixing the water in a fish’s transport bag with their new tank. This doesn’t take into account predators that may want them for a snack. If your goldfish is lucky, they may escape death or predation and actually grow into one of the giants. Is this okay? Unfortunately not – remember that goldfish are native to China, and not naturally found elsewhere. This makes them an invasive species if they live elsewhere.
Goldfish living where they shouldn’t be have been known to damage delicate water plants, outcompete native species by eating all their food (or the native species themselves!), and spread exotic diseases. Some parts of the world have over 30 non-native fish living in their waters. Never put a live fish anywhere that it could get into the watercourse, including flushing them down the toilet! Keep them in a well-maintained tank where they belong, and your hard work will reward you with a healthy happy fish, whatever their size.