Raising Koi Fry: From Hatching to Juvenile

By KoiQuanta Editorial Team|Updated Mar 2026

Koi fry culling at 4-6 weeks is one of those practices that sounds harsh until you understand why it exists. In a quality koi breeding operation, culling isn't optional. It determines the genetic quality of every variety you produce. Without it, you're not breeding koi. You're just producing fish.

But culling is only one part of raising fry successfully. Before you get to week 4, you have to keep them alive through the most vulnerable phase of their lives. This guide covers the full first six months, from the moment they hatch to juvenile fish ready for grow-out ponds.

TL;DR

This guide covers the first 6 months from hatching to juvenile stage.
Before you get to week 4, you have to keep them alive through the most vulnerable phase of their lives.
For the first 2-3 days after hatching, they stick to surfaces (the side of the tank, plants, your hands if you're unlucky enough to put them in) and absorb their yolk sac.
Feeding frequency at this stage: 4-6 small feeds per day.
Change 10-15% daily rather than less frequent larger changes.
Feed transition: - Continue brine shrimp as the main food - Introduce powdered or crumbled commercial koi fry pellets - Begin offering micro-pellets by week 3 as mouths get larger Feed 4-6 times daily.
If fry are eating every particle within 30 seconds and actively searching for more, increase the amount slightly.

Week 0-1: Hatching and Yolk Sac Stage

Newly hatched koi fry are tiny. 5-7mm and nearly transparent. For the first 2-3 days after hatching, they stick to surfaces (the side of the tank, plants, your hands if you're unlucky enough to put them in) and absorb their yolk sac. They don't feed during this period and shouldn't be fed. The yolk sac provides everything they need.

Keep the hatching tank:

Gentle aeration. Not strong enough to tumble the fry.
20-22°C (68-72°F)
Clean water with no ammonia
Away from strong light initially

By day 3-5, fry become free-swimming. You'll see them horizontal in the water column rather than hanging from surfaces. This is when feeding begins.

Week 1-2: First Feeding

Starting fry feeding correctly sets up growth rate and survival through the critical early weeks.

First foods (days 3-10):

Infusoria (microscopic organisms found in green water)
Commercial liquid fry food
Hard-boiled egg yolk strained through fine cloth and diluted heavily. Use sparingly; it causes koi pond water quality tracker problems if overfed.
Green water (phytoplankton culture) as both food and water conditioner

At this stage, fry mouths are tiny and they can only consume microscopic particles. Commercial liquid fry foods designed for cichlid or koi fry are reliable and don't cause the water quality crashes that egg yolk can cause.

By days 7-10: Introduce newly hatched brine shrimp (Artemia nauplii). This is live food that moves and triggers feeding behavior. Fry growth accelerates noticeably with brine shrimp introduction.

Feeding frequency at this stage: 4-6 small feeds per day. Fry have tiny stomachs and can't store food. Frequent small meals support faster growth than two large ones.

Water Quality During First Feeding

Water quality management is the hardest part of fry raising. You're feeding heavily into a small tank. Ammonia builds fast. But fry are too fragile for aggressive water changes, and siphoning incorrectly kills them.

Use a siphon tube with a fine mesh sock or cloth over the end to prevent sucking up fry during water changes. Change 10-15% daily rather than less frequent larger changes. Test ammonia daily. Above 0.25 ppm, change more water.

Sponge filters are the best filtration choice for fry tanks. Gentle flow, no suction risk, and a surface that grows beneficial bacteria without the risk of conventional filter intakes.

Week 2-4: Rapid Growth Phase

This is when you start seeing real growth. Fry grow fast. Under good conditions, they'll double in size in two weeks.

Feed transition:

Continue brine shrimp as the main food
Introduce powdered or crumbled commercial koi fry pellets
Begin offering micro-pellets by week 3 as mouths get larger

Feed 4-6 times daily. If fry are eating every particle within 30 seconds and actively searching for more, increase the amount slightly. If food remains after 2 minutes, you're feeding too much.

Watch for the first major threat at this stage: the emergence of fast-growing individuals that start eating slower siblings. This happens earlier than you'd expect, sometimes by week 2. These "bullies" need to be separated or the size variation in your cohort will compound.

How to Keep Koi Fry Water Clean

The challenge is feeding enough to support fast growth without crashing water quality. Here's the approach that works:

Feed small amounts 4-6 times daily instead of large meals
Siphon the tank bottom daily to remove uneaten food and waste
Do 10-20% water changes daily, matching temperature precisely
Dechlorinate all new water before adding it
Run an air-driven sponge filter continuously
Don't use UV sterilizers or chemical treatments unless actually needed. Fry are sensitive.

Once the fry grow large enough (around week 3-4) to handle more vigorous water flow without being swept around, you can increase filter capacity.

Week 4-6: Culling

This is the most consequential management decision in koi breeding, and the one most hobbyists skip or do badly.

What Culling Is and Why It Matters

Koi genetics don't breed true with predictability. A pairing of two excellent Kohaku parents produces a wide range of offspring. Some with good pattern potential, many without, and some with genetic defects. The purpose of culling is selecting only the best fish to continue growing and eliminating the rest.

"Eliminating" means humanely euthanizing: clove oil is the accepted standard at 0.1-0.3 mL per liter until the fish is unconscious, then a higher dose for euthanasia, or other vet-approved methods. Don't release unculled koi into natural waterways. Don't sell every fry indiscriminately if you care about the quality of the variety you're breeding.

What to Cull At 4-6 Weeks

At this stage, fry are 2-4 cm. You can clearly see:

Body shape defects: Spinal curvature (scoliosis), bent bodies, stunted growth
Obvious fin deformities: Fused, missing, or malformed fins
Early color indication: For most varieties, the pattern is not yet visible. But you can see which fish are entirely dark (future Magoi-type fish unlikely to produce show quality).
Size outliers: Extremely stunted fish that haven't grown at the same rate as siblings often have underlying developmental issues

Keep the fish that show good body confirmation, uniform growth, and no physical defects. The number you keep depends on your grow-out space and target.

A typical cull at 4-6 weeks retains 20-30% of the fry cohort. Some breeders are more selective, retaining as little as 5-10%.

Week 6-16: Grow-Out Phase

After culling, surviving fry move to grow-out conditions. This typically means a larger pond or tank where water quality is easier to maintain and growth can continue.

Feeding at grow-out stage:

High-protein pellets appropriate to mouth size (1-2mm pellets initially)
3-4 feedings daily
Target feeding until fish are satiated but not so much that food remains uneaten

Transition fish gradually between feeding amounts and locations. Sudden stress at this stage, from handling, transportation, or major water changes, can trigger disease.

Second Cull at 8-10 Weeks

By this stage, variety patterns are becoming visible on most koi types. This is when Kohaku keepers can start seeing where hi (red) is developing, where Sanke tri-color markings are appearing, and which fish show the most promise.

Second cull criteria:

Eliminate fish showing pattern development that clearly won't meet variety standards
Eliminate fish with progressive size lag compared to siblings
Select for good body shape and fin quality in fish where pattern is good

Month 3-6: Juvenile Development

By month 3, surviving fish are clearly juveniles. Recognizable as small koi. They should be on regular koi pellets appropriate to their size, fed 2-3 times daily.

Key management priorities in this period:

Monitor for parasites regularly. Gill flukes and skin parasites establish easily in young fish.
Watch for bacterial infections in any fish with minor injuries
Continue sorting by size to prevent larger fish from outcompeting smaller ones for food
Begin photographic records of each fish if you're tracking for quality selection

When to Move Juveniles to the Main Display Pond

Juveniles are ready for main pond introduction when:

They are large enough that pond equipment (bottom drains, skimmer inlets) won't pull them in
They can compete for food at the surface with larger pond fish
Water quality in the main pond is confirmed stable
You've done any prophylactic parasite treatments on the juveniles before introduction

For most grow-out conditions, this happens at 3-4 months at the earliest, often 5-6 months.