Koi are known for being large fish. A fully grown koi can reach upwards of 24” or more, which is why it can be worrisome as a hobbyist when your koi’s development is stunted. If your koi aren’t growing, there’s likely a reason for it.

It’s important to keep in mind that there are a number of things that impact fish development. Here are six common factors that can impede koi growth and a few ways you can fix the problem.

Reason #1: Poor Water Quality

If your pond environment isn’t clean, it can cause an enormous amount of stress for your koi. Over time, stress can lead to a number of other problems, including decreased growth. Keep in mind that just because your water looks clean doesn’t mean it is clean. There are plenty of invisible pollutants that can build up in your pond over time and negatively impact your koi.

The simplest way to ensure your pond water is healthy is by consistently monitoring water parameters, performing regular water changes, and making sure your pond stays free from waste and other debris. If organic material and other waste begins to breakdown in the water, it can raise ammonia, nitrate, and nitrite, which can be extremely harmful to your koi. You can monitor these levels, along with pH, using a water test kit. You can learn more about the nitrogen cycle and maintaining your pond water in our article The Shape of Koi Water.

Make sure your koi are getting plenty of oxygen. If your fish are crowding around oxygen sources or gasping for air at the surface, that’s a telltale sign that the dissolved oxygen (DO) levels in your pond are low. Ideally, the DO levels should be around 7-10 mg/L. You can monitor these levels by purchasing a dissolved oxygen meter. If levels are low, creating more water movement by adding a water feature or air pump is an easy way to increase DO. Properly maintaining the water quality in your pond will help ensure your koi have the best environment to grow.

Reason #2: Water Temperature

Koi cannot control their own body temperature. They depend on the water temperature in the pond to regulate their metabolism. Warmer water temperatures increase their metabolism and results in higher activity levels and appetite, which typically leads to growth. However, water that is too warm can cause stress and illness, which will reduce growth. On the other hand, if the water temperature is too cold, koi will begin to enter a state of hibernation, which causes decreased appetite, lethargy, and decreased growth. This is normal in colder climates, and it’s typical for koi growth to lessen in the winter months.

The ideal water temperature to maintain a steady growth rate is between 65°F and 75°F. Water temperatures can be monitored using a sinking or floating thermometer. If your pond water is too warm, the best solution is shade. Installing a shade sail over your pond or adding a few pond plants can help cool things off. Temperatures that are too low are typically the result of season changes, which are completely normal. You can always consider bringing your koi inside for the winter if you want them to keep growing during colder months.

Reason #3: Overcrowding

Fish need room to grow. If your pond is too crowded, it can stunt the growth of your koi. Koi often have to compete for food in overcrowded ponds, which creates an extremely stressful and hostile environment. An overcrowded pond also means more waste, which can cause water-quality issues to arise more quickly.

To keep your koi happy, your pond should hold a minimum of 1,000 gallons of water, have 10 square feet of surface area per koi, and 10 gallons of water per every one inch of fish. Unless you want to rehome a few of your koi, there’s really only one solution for overcrowding: you’re going to need a bigger pond.

Reason #4: Pond Size

Even if your pond isn’t overstocked, a pond that is too small can also cause issues. Ponds with little space limit your koi’s ability to exercise and build muscle and make your koi more susceptible to predator attacks. These space limitations can be stressful for your koi, which can hinder their growth. Water quality and temperature fluctuations are also more common in small ponds.

The more space there is for your koi to move around, the happier they’ll be. Keep in mind that size doesn’t just mean length and width, it also includes pond depth. Deeper ponds typically have better water quality and make it easier for your koi to hide from any predators that may come lurking. Read more about pond depth in our article How Deep Should Your Koi Pond Be?

Reason #5: Poor Nutrition

Diet is a key factor in koi growth. A poor diet will not only stunt growth, but it can also cause stress and illness. You want to avoid low-quality foods that are high in carbohydrates and fat, which can be difficult to digest and often lead to disease.

Make sure you are feeding your koi a high-quality feed that is made specifically for koi. Choose a feed that is high in protein and vitamins or a formula designed to promote growth. We recommend Blue Ridge Growth Koi and Goldfish Food, because it is specially formulated to enhance growth in koi and goldfish. Also keep in mind that your koi’s diet and feeding schedule should be adjusted as water temperature changes and their metabolism begins to slow.

Reason #6: Genetics and Age

Sometimes reduced growth is caused by factors out of your control, such as genetics and age. Some koi just naturally grow more or less than others. It’s also important to note that koi grow the most when they are young, typically within the first 2-3 years of life. If your pond environment is in good shape and your koi are healthy but still not growing, it’s likely because of genetics or age, meaning there really isn’t anything you can do about it.

Stress is the common thread with most of the factors influencing growth. Overcrowding, small pond size, poor nutrition, water quality, and changes in water temperature can all cause stress. If your koi are stressed, their immune systems are weakened, and they become more susceptible to illness—which can lead to decreased growth. The most effective way to ensure that your koi grow is to provide them with the best possible environment and nutrition.

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