How to Grow Big Koi
Did you know that the size to which a koi can grow is practically unlimited? Jumbo fish have clocked in at 30 inches or more, and they can weigh 20+ pounds. There have even been Gosanke varieties that have exceeded the meter mark.

Koi hobbyists are always looking to grow bigger fish, and a healthy environment paired with proper care can help a four-inch koi reach a length of two feet.

Getting Your Koi to Grow Big

  1. Genetics
    It all starts with genetics, the predetermined physical traits of the fish, expressed as a phenotype. If the fish is not predetermined to grow big, it won’t, regardless of the keeper’s relentless attempts. The goal, of course, is to get each fish to reach its genetic limitations. If properly tended, most koi will grow to at least 20 inches in adulthood.To maximize genetics, choose a young female with a larger head, backbone, and overall frame. It’s important to know the fish’s bloodline; larger parents typically mean larger offspring. Generally speaking, solid-colored fish, including Ogons, Chagoi, and Magoi, tend to grow larger than Gosanke, a more inbred variety.
  2. Age
    The younger the fish when it joins your pond, the better your chances of getting it to grow large. If your goal is a large fish, you’ll need to start the process in the first two to three years of its life.
  3. Water Quality
    The water in which the koi grow is a pivotal component in their growth. As koi hobbyists, we are water keepers first and fish keepers second. Your focus on your water will show itself in your collection. If, for instance, your water has chronic levels of ammonia and nitrite, you’ll hinder growth and likely cause a disease state. Nitrates in excess also result in slowed growth, and pH and oxygen saturation need to be stable. When your water is chemically clean, it will allow your fish to grow and thrive.
  4. Water Temperature
    Although koi can adapt to multiple temperatures, you’ll see optimal results in optimal water temperature. The standard environmental temperature (SET) should be 25–27ºC. At this temperature, metabolism will accelerate, resulting in fish being able to eat more food and grow faster.Naturally, in the winter months, the pond will cool, but you should strive to keep that period short (no more than two to three months) with temperatures around 10ºC. New members to your pond need to winter in warmer water or their growth could be stunted. Remember that koi complete most of their growth cycle before sexual maturity, which is two years for a male and three years for a female.
  5. Proper Nutrition
    As with any other living creature, proper nutrition is key to sustained growth. Since fish graze and have less than efficient digestion, it’s a smart idea to offer six or more meals daily. Koi are primarily vegetarian so should be offered dark green, leafy vegetables. Provide a diet that is fresh, full of nutrients, and offers variety to be successful.
  6. Large, Deep Pond
    When everything else is running smoothly, it all comes together in the pond. Fish tend to grow best in low-stocked, high-water-volume mud ponds that allow them to exercise to develop muscles while providing plentiful natural food. A pond such as this mimics a natural environment with the added bonus that fish don’t have to waste energy looking for their food since it’s readily available. While a smaller pond could benefit from jets, it still won’t be as perfect an environment as a mud pond.

Putting It All Together
Each of these components is critical separately and together when large koi are the goal. Some varieties can add four to six inches a year when conditions are in their favor. While many start to slow their growth at age five, others will continue to grow for up to a decade.

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Photo by Bernard Spragg