How to Care for & Raise Koi Fry (Growing Tips)

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Koi swimming underwater
It’s important to provide your koi fry with optimal care conditions, as they can eventually turn into highly valuable fish. R’lyeh Imaging / CC BY 2.0

If your pond is stocked with mature male and female koi, you may at one point be faced with the enjoyable dilemma of rearing koi fry. There’s nothing quite as rewarding as seeing prized fish go through the stages of their life cycle and develop into beautiful adults. This is partly why koi breeding is so popular. Breeders revel in the growth process of young fry, which eventually turn into highly valuable fish under optimal care conditions.

To successfully breed koi, you will need to prepare for each major phase of their life cycle. It is advisable to familiarize oneself with spawning preferences, signs of koi “pregnancy”, and egg hatching requirements so that you may properly anticipate the arrival of koi hatchlings. As koi eggs hatch just a few days after fertilization, koi survival rates may hinge on the quality of care received in such a short time frame.

To increase survival rates and keep a close eye on the baby fish, consider making use of a separate tank setup.  This article will take you through all of the important aspects of rearing koi fry so that you can prepare for their needs in a timely manner. If all goes well, you get to count yourself lucky enough to experience every koi lover’s dream.

Physical Features of Koi Fry

From just 40 hours after fertilization, develop begins to become visible in koi eggs.

Upon hatching, koi fry are virtually impossible to tell apart from the hatchlings of other fish species. They appear as skittish wrigglers, weighed down by their yolk sacs and unable to swim due to the absence of a swim bladder and fins. They begin to swim after 2 – 3 days, once their mouths, fins, and organs have begun to develop.

At this stage, you’ll find that they begin to work their way towards the water’s surface. As they take up their first few gulps of air, their swim bladders start to expand and regulate buoyancy. They have no pigmentation at this stage. It may be quite challenging to peer at individuals without a magnifier, as they grow to just approximately 7 mm once they lose their yolk sacs.

Koi fry underwater
You may need a magnifier to look at koi fry, as they grow to just 7 mm! Adam / CC BY-SA 2.0

Koi fry gradually increase in size once they begin to feed. Keep in mind that a fair number of newly hatched fry may fail to reach this stage, as some may develop mouth or fin malformations. Dead fry and any unhatched eggs should be removed as they can encourage fungal and bacterial growth. In the wild and in outdoor ponds, any surviving fry will likely be seen close to the fronds of submerged or marginal plants.

Koi Fry Growth Timeline & Selection Process

Koi fry in a person's hand
Koi begin to show colors and patterns as early as 2 weeks after hatching. Photo from hippopx

Regularly fed fry will grow at a rate of about 0.66 mm per day. This means that after 4 weeks of growth, healthy fry should be about 2 cm long (a little less than an inch). After two months, your fry should be about 5 cm (2 inches) long. You can make use of a coin with a similar diameter to check on the growth rate of your fry. Those that have stunted growth rates are unlikely to make it past the 4-week mark. If they do, you may have to separate them from your healthy fry.

When do Koi Fry Get their Color?

Koi begin to show patterns and colors as early as two weeks from the hatch date. This is such an exciting period of their growth timeline as you will likely begin to single out individuals that hint at desirable colors and patterns. Of course, these colors are subject to change over time. They may grow deeper or fade as the fish grow. You should not select for colors or patterns at this stage and should instead place more weight on normal organ development and overall growth.

Five weeks after your koi begin to swim to the surface, you must peer closely at each individual and begin your selection process. It may be tempting to simply leave all fry in the tank and let nature take its course. However, allowing stunted individuals to grow alongside healthy ones may, unfortunately, do more harm than good. Deformed and weak fry tend to have compromised immune systems and may contract potentially transmissible diseases.


Continue to pay close attention to your fry population, removing those that are unable to catch up in size with the larger fry. Once you are satisfied with the health of your remaining fish, you can begin grading them. As they increase in size, your rearing tank or pond will comfortably accommodate less and less fish. This is why you must keep only the best and either place the rest in other tanks, give them to friends, sell them, or even cull them if the need arises.

This part of the selection process would of course be near impossible in an outdoor fish pond, but you can rely on natural pressures to sort the strong from the weak – that is, if the entire fry population doesn’t get wiped out.

What to Feed Newly Hatched Koi

Female adult daphnia
You can serve ground-up frozen daphnia, among other things, to baby koi. Dunk / CC BY 2.0

Koi fry should not be fed until about 2 days after the hatching period. When attached to their yolk sacs, the hatchlings do not yet have developed mouths and will be unable to take up food. Instead, they persist on the nutrient stores in their sacs. Their mouths and swimming organs develop as the yolk sac becomes depleted. Fry swimming to the surface is usually a sign that they may be ready to take up food as their mouths, fins, and swim bladders develop at around the same time.

The first few meals of hatchlings will consist of microbes (infusoria) that are naturally found in the water column. If you have elected to rear your fry in a tank, their diet will need to be supplemented with protein-rich food to facilitate normal growth and development. Consider creating a makeshift hatchery for brine shrimp as their larvae are simply the best source of protein for fry.

If you require alternatives that can be store-bought and provided instantly, there are many convenient food options. Commercial fry starter formula can be purchased from fish shops and their online portals. Just make sure to purchase enough to sustain your little critters for the first few weeks. You definitely don’t want them starving for food as their organs develop.

Fish flakes or koi pellets that are ground into a powder should also sustain your fry and can be provided as early as week 2, when they begin to look more like actual fish! Freeze-dried krill or frozen daphnia can also be ground up and served to baby koi.

Feeding Timeline for Koi Fry

A group of koi competing for food
It’s best to raise baby koi in a separate tank. If they’re in a pond with mature koi, feeding them can lead to competition, endangering them in the process. Bernard Spragg / No copyright

Below is an outline with feeding suggestions and frequency. You’ll have to make use of the trial-and-error technique to determine how much food is enough. If you feed your fry with too much food, leftovers can quickly pollute the tank. Conversely, food shortages can cause many fish to starve and impair organ growth. Always try to feed in small increments and increase the feeding frequency as needed.

  • Days 1 – 2 (newly hatched koi) – Do not feed your fish for as long as their yolk sacs are present.
  • Day 3 – Provide homemade or commercial liquid food, such as a fry starter or egg paste (crushed hardboiled eggs with ½ cup of water per yolk); feed 4 times/day.
  • Days 4 – 5 – Add brine shrimp larvae and increase feeding portions as needed.
  • Days 6 – 7 – Experiment by adding powdered food to their diet.
  • Week 2 – Continue providing your fry with a mix of egg paste and liquid food. Add in crushed pellets. Daphnia can also be provided at this time. Feed 4 times/day.
  • Weeks 3 – 4 – Reduce the amount of egg paste and liquid food until you are able to replace them completely with krill and fish flakes.
  • Month 2 – Ensure that your fry receive a well-balanced diet. You can combine the following options per feeding time or alternate throughout the day: crushed brine shrimp or daphnia blocks, sinking pellets (formulated for fry), crushed pellets for adult koi, pulverized fish flakes. Feed 4 times a day; food should be consumed within the first five minutes of feeding.
  • Month 3 – Reduce frozen daphnia or brine shrimp and increase the amount of pellets or flakes. Feed just 3 times a day and make sure to remove excess food.
  • Month 4 – Feed as you would juvenile koi. Feeding frequency and times should be adjusted according to season if you intend to introduce your koi to an outdoor environment. Keep in mind that juveniles will still require a higher feeding frequency than adults.

If your fry must be reared in the fish pond, they will search for natural sources of protein. You may attempt to supplement their diets with ground-up flakes or pellets, but this may prove to be challenging as you will have to locate them first. Moreover, this can draw in other fish that may compete with them for food and endanger them in the process. This is why rearing baby koi in a separate tank is always a wise decision.

Koi Fry Optimal Tank Conditions

An air pump and air stone for a tank
You must provide oxygen in your tank with the use of air stones (pictured, right). Ofkun, CC BY-SA 4.0, via Wikimedia Commons

In a tank, the feed consumption and growth rates of fry can easily be monitored. To keep survival rates high, physical parameters must be kept at optimal levels. Professional breeders monitor tank water on a daily basis so that adjustments can be made before a system collapse. They have advanced equipment for these purposes, but for your own home-based tank you will simply need a water thermometer, pH meter, and test kits for ammonia, nitrates, and dissolved oxygen.

During the first few days of life, it is inadvisable to switch on your tank’s filter as the baby fish may get sucked in. A sponge with very fine pores or a fine mesh net may be used as a barrier, but these will have to be cleaned thoroughly and frequently. Once the fry are able to take up food, a filter becomes indispensable as any decomposing food particles can compromise the water quality. Make use of one that is recommended for the specific size of your tank. If you’re reluctant to use a filter, you will have to do frequent water changes.

Oxygen must be provided with the use of air stones, which fry typically gravitate toward. Air stones must be monitored frequently as any malfunctioning ones will need to be replaced or fixed before oxygen levels drop. Optimal levels for easily manageable parameters are listed below.

  • Temperature: 65 – 75˚F (18.3 – 23.9˚C)
  • Ammonia and nitrite: 0 ppm
  • Dissolved oxygen: at least 6 – 7 ppm
  • pH: 7.5 – 8

Prevent Overcrowding Koi Fry

Orange & black baby koi in a person's hand
It’s more sensible to be fully dedicated to only the best growing fry (in terms of color, pattern, growth rate, etc.). Image by Giovanni Bagayas from Pixabay

At the end of the first month, you may need another tank to accommodate your growing fry. Each baby koi will require more and more space as it grows. The carrying capacity of a single tank may be unable to support hundreds of thriving fry. Even if conditions are optimized all throughout, high densities can result in slowed growth and the starvation of many fish.

Breeders usually prevent overcrowding through several rigorous selection phases, whereby any fish that doesn’t meet the cut (in terms of color, pattern, health, and growth rate) is either culled or sold to mass buyers. Even if you have the means and time to care for hundreds of growing fry, it makes more sense to be fully dedicated to the best of the stock. Keep in mind that, if you would like to eventually transfer your fish to an outdoor pond, you must consider their requirements in terms of water volume.

How to Transfer Juvenile Koi to Your Main Pond

Juvenile koi in an aquarium with mature koi
Your juvenile koi may shy away from larger fish at first, but with time they’ll gain confidence! Raita Futo / CC BY 2.0

After 5 – 6 months have passed, your koi should be large enough to transfer to your main pond. To ensure that they survive the move, they must undergo a brief acclimation period. The steps are exactly the same as those for fish that you purchase from pet stores. If the main pond is located outdoors, this acclimation procedure should only be done when ambient temperatures stay above 70˚F (21˚C).

First, select the koi you wish to transfer. Group the selected fish and place them in a quarantine tank. Prep them for the transfer by gradually altering the pH and temperature in their tank until the values are very close to those of your outdoor pond. There should be a difference of, at most, 0.2 for pH and 5˚F (2˚C) for temperature.

Once the juveniles are accustomed to these levels, use a fine net to individually bag your koi. Each bag should have enough water to comfortably cover the fish. The rest of the space should be filled with oxygen prior to closing up the bag. Carefully take the bagged-up fish to your pond and allow them to float on the water’s surface. The best time to do this is early to mid-morning when sun exposure is moderate.

It should take about 20 minutes to an hour for your fish to acclimate to the pond’s temperature. Gently release your young koi in a section of the pond where they are afforded some protection. Keep a close eye on them during their first few days in the pond. They may shy away from larger fish and attempt to hide. This is normal behavior, but you’ll find that they will soon grow comfortable exploring all areas of your pond!

Chris G
About the author

Chris G

Pond consultant and long-time hobbyist who enjoys writing in his spare time and sharing knowledge with other passionate pond owners. Experienced with pond installation, fish stocking, water quality testing, algae control and the troubleshooting of day-to-day pond related problems.

Read more about Pond Informer.

4 thoughts on “How to Care for & Raise Koi Fry (Growing Tips)”

  1. Hi thanks for that information I have a very large dam and have fry for the first time. What would eat my fry? I have water snakes lizards turtles ducks eels all in my dam will my young survive?

  2. returning home from a 2 week vacation, I found our first hatching of approximately 30 live baby koi. I have fed them crushed pellets but they appeared to be 2 weeks of age then and now approximately 1 month. They have all survived and growing rapidly. we have 4 adult koi and now 35 babies. When can I remove most of them and sell or give away?

  3. Great article. I have a three tiered pond with koi only in the top pond. Nature just finds a way. We were surprised to find 40+ fry migrated on their own to the low pond. They’re growing well. Now what to do with 40+ new koi!!


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