Why Is My Koi So Small?

A tranquil pond with a beautiful collection of majestic Koi fish is the pride and joy of many homeowners. The peaceful atmosphere that Koi creates is a blessing to any home. But for many first-time pond makers their beautiful Koi fish remains small.

Factors such as stress, poor nutrition, and the wrong pond conditions, such as temperature, overcrowding, and low oxygen levels, may cause your Koi fish to be undersized. Bacterial infections and stress caused by predators may also be factors.

Knowledge is vital as there may be many reasons why your Koi are not growing as expected. Making sure you apply the correct care to your Koi Pond, understanding the growth pattern of your Koi, and providing the proper nutrition is key to making sure that your Koi grow big and strong!

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What Is The Correct Size For A Koi Fish According To Age?

If you are a first-time Koi-keeper, it may help to understand the natural growth trajectory of a Koi fish so that you know what is normal and whether your Koi may be too small for its age.

 All Koi fish species can grow up to 24 to 36 inches long by the time they are adults. If you happen to have a jumbo Koi fish, you can expect them to reach up to 52 inches.

The growth trajectory of your Koi starts with a young Koi only measuring 1.1 inches at around the one-month mark. At six months, your Koi would measure around 5.1 inches. If you have had your Koi for a year already, you should be seeing sizes of up to 9.2 inches.

If your Koi fish stopped growing after a year, other factors might be in play, such as water quality, proper diet, or overcrowding.

After the one-year mark, the growth trajectory of your Koi fish speeds up quite a bit.

Check the growth chart below to see how big your Koi should be at any age and whether they are very far off from what is considered a typical growth trajectory.

18 Months – 12.6 inches

2 Years – 15.6 inches

2.5 years – 18 inches

3 years – 20.1 inches

3.5 years – 21.6 inches

4 years- 23.4 inches

4.5 years -24.6 inches

5 years- 25.7 inches

5.5 years -26.6 inches

6 years- 27.3 inches

6.5 years- 28 inches

7 years -28.5 inches

Do Some Koi Fish Varieties Stay Small?

It is possible that what you have in your pond is not a Koi fish at all but a Shabunkin! Shubunkin fish are known as the poor man’s Koi for a reason. They have beautiful colors arranged in a calico style, and they thrive in all the same conditions that a regular Koi fish would.

These beauties are no more than giant goldfish, and although they have the colors typically seen in Koi fish, they do not display the patterns seen in different Koi varieties. Shubunkins can grow up to 20 inches in length.

Does The Wrong Food Stop Koi Fish From Growing?

Poor nutrition is a significant cause of under-developed Koi fish. It may be tempting to go for cheaper food, especially if you have several fish to feed but trust me, it is not worth it!

Not only can poor nutrition affect the size of your Koi, but in many cases, it can lead to overall poor health or even death!

Ensure that your Koi feed contains enough quality protein to aid in growth. Always look for food that has at least a 35 percent protein content. Roughly 30 percent of the feed should consist of high-quality carbohydrates, which will help give your fish the right amount of energy.

Also, make sure that your feed contains Lipids, these oils are essential, and your feed should ideally consist of around 10-12 percent. Look for added Vitamins C and E as this helps retard free radical damage and keeps your Koi feed fresher for longer.

Avoid any Koi food that contains a high number of artificial preservatives, colorants, and flavorings. Avoid fillers that act as a substitute for protein.

Does Stress Stop Your Koi From Growing?

Yes, Koi fish do experience stress. Koi are incredibly sensitive creatures and need a very stable environment to thrive. Signs that your Koi fish may be experiencing stress include low energy, poor appetite, and the tendency to hide.

You may start to notice changes in the appearance of your Koi fish, such as duller colors, damaged scales, and injuries to their fins. Changes in your Koi’s appearance may be due to the presence of predators, parasites, or bacterial infections.

Be on the lookout for natural predators like herons, cats, hawks, and even raccoons in the environment that could be trying to have your Koi for lunch. Finding ways to deter these predators could help lower the amount of stress your Koi has to endure.

Another way to help with stress caused by predators is to include plants and features in your pond that give your Koi fish a place to hide. With no place to seek refuge, your Koi may become increasingly stressed, causing slow growth.

Predator deterrents that you can try would be installing a sprinkler system or covering your Koi Pond with netting.

The Role Genetics Play In Keeping Koi Small

You need to understand that genetics play a significant role in a specific Koi’s average size. If they come from a lineage where both mom and dad were undersized, the chances are that they will not grow significantly larger than their parents.

A smaller size does not mean that your Koi are not happy and healthy; it just means that they will grow on the lower scale of the average length and weight for their age and what is typical for their species and individual genetics.

Koi Pond Conditions That Keep Your Koi Small

Water Quality


Creating the right water conditions for your Koi will help ensure that they continue to grow big and strong. Providing the correct oxygen levels ensures that the good bacteria will thrive.

Low oxygen causes stress as Koi start to pick up bacterial infections that will retard their growth and even result in death. There are some practical ways to improve the oxygen levels in your pond, which include adding natural aerators like waterfalls or fountains.

Another way to improve the oxygen level in the pond is to install an air pump. Adding plants, such as Arrowhead (Sagittaria subulate), Eelgrass (Vallisneria), and Fanwort (Cabomba) to your pond will naturally increase the oxygen level.

Bacterial And Parasitic Infections

 Dirty water, incorrect pH levels, high levels of ammonia, and fluctuating water temperatures can cause bacterial and parasitic infections. Bacterial infections, like Fin Rot, Ulcers, Mouth Rot, Dropsy, Popeye, and Tail Rot, can cause white cotton-like spots on the scales.

Parasites such as flukes and tapeworms may be challenging to spot, but you will notice that your Koi fish fins become frayed and that they start to appear much duller in appearance.

Make treating your pond water with anti-bacterial and anti-parasitic treatments part of your regular maintenance routine, as Koi generally respond well to these.

Size Of The Pond

As a rule, the lower the number of Koi in your pond, the larger their size will be. To conserve resources and ensure that each Koi fish has an optimal chance of survival.

Koi fish naturally regulate how many new Koi fish get introduced into their environment.

 Koi fish can’t control how many other fish they share their space within captivity. This not only causes stress but will ultimately result in smaller-sized Koi.


The correct pond size for Koi fish would allow 250 gallons of water per Koi fish. Cramming too many Koi fish into a pond is dangerous as oxygen levels will drop and cause your Koi to experience stress, which in turn inhibits growth and leads to bacterial infections.


Getting your beautiful Koi fish to grow big and healthy will require that you pay careful attention to their environment and provide high-quality nutrition. Regular treatments to avoid bacterial infection and keeping your pond clean will ensure that your Koi will flourish in size.

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