Goldfish produce a growth inhibiting hormone (GIH) that builds up in the water. A bigger tank helps to dilute this hormone, which is why goldfish tend to get really big in a big tank. Common Health Problems. Goldfish can grow their whole life. Goldfish sold as "tank suitable" will grow to about 10 inches while those sold as "pond suitable" will reach a maximum of about 18 inches. According to DEC when Goldfish are kept as a pet in small in a small aquarium, they can get bigger as much as 6 inches but wild Goldfish can get about 13 to 14 inches long. They can grow up to 12 inches when fully matured. Commons and comets get 12-18 inches long. You should expect them to live from 4 to 14 years. The fish belonging to a lady from the Netherlands has never been weighed, so its mass remains unclear. How big goldfish get in the wild? Koi get 36+ inches long. How Big Do Comet Goldfish Grow. Like other Goldfish, the Comet is prone to some common health problems including: When that water is changed all the time, the hormone is removed and the fish continues to grow. Because there are many different species of goldfish, it is not always easy to answer the question, “How big do goldfish get.”The first thing you need to think about is the fact that goldfish are bred from carp. The goldfish caught by Lois Chilvers is the largest goldfish in the world. All of our fish in our pond are about a foot long except for the younger fish. Goldfish kept inside in aquariums should live 10+ years. Now: How many goldfish can you keep in a 20 gallon tank? If goldfish are kept outdoors in a pond, attain a good size and are fed the proper diet, 20 years is not uncommon. A: Goldfish kept in a bowl have the shortest lifespan, 5 years being exceptional. There are three factors to consider when it comes to the size of goldfish: The variety and genetics: common goldfish can easily reach 25-30 centimeters (10-12 inches) in length but other smaller varieties are only half of this.More information on the most common goldfish varieties on this page How big will my goldfish get? A good filter, with no heater, is recommended because these fish can get larger than 12 inches (300 mm). You’ll find the answers in our guide to goldfish tank size.. Of course, you’ll also need to know how big your own tank is. I have seen a koi at a pond store that was easily 3 feet long. To date, the longest goldfish was 18.7 inches. In China goldfish 40+ years old have been recorded. But if you maintain a good size of the aquarium for Goldfish, it can get bigger up to 13 to 14 inches even in aquariums. Size. Carp are actually a family of freshwater fishes and some of the more common species can grow anywhere from 16 to 30 inches in length and can weigh upwards of 25 pounds. Fancy goldfish get about 6-8 inches long but can get longer. While not officially acknowledged by the Guinness Book of World Records, the goldfish that this proud 10-year-old caught broke the record for the largest goldfish caught in the UK. How much space do common/comet and fancy goldfish need? Genetics are the predetermined factor in how big your fish will grow. The size of your tank should be based on the size the fish will be when he is completely grown. For every small/young goldfish, there should ideally be at least 10 to 20 US gallons (38 to 76 l; 8.3 to 16.7 imp gal) of water. Shubunkin and sarasa goldfish get about 12 inches long. The wide lifespan accounts for the range of common illness which they can experience. The fish, which belonged to a Dutch lady, was never weighed, so his mass remains unclear. Comet Goldfish Lifespan. However, a goldfish caught in Michigan measured 15 inches and weighed 3 lbs, so it’s safe to assume that Dutch behemoth weighed in excess of 3 lbs. An experiment done over 30 days showed goldfish kept at 79 o F (26 o C) weighed twice as much as goldfish kept at 70 o F (21 o C) at the end of the 30 days. Q: How long do goldfish live? How about a 30 gallon tank? Water temperatures have a huge impact on goldfish growth rates. The longest goldfish to date measured 18.7 inches. How big are goldfish? Growth Rates From Week Nine to One Year Lois caught her prize specimens at Chase Lakes during a fishing weekend with her father Gary. A goldfish caught in Michigan, however, measured 15 inches and weighed 3 lbs, so assuming that Dutch behemoth weighed more than 3 lbs is safe. Tank recommendations range up to 75 US gallons (280 l; 62 imp gal).