My eggs turned completely white. Will they hatch?

No. White eggs are infertile or have stopped developing. Remove them quickly so fungus doesn't spread to the healthy eggs.

How many eggs does a medaka lay?

10–30 per spawning, and they spawn almost daily in season (early summer to fall). One female can produce several hundred eggs in a season.

The parents keep eating the eggs. How do I stop them?

You can't train them, so separate the eggs instead. Collect eggs in the morning (spawning peaks at dawn), or use a spawning mop you can lift out whole.

None of my eggs are ever fertile. What's wrong?

Usually one of three things: no functioning male (check the fins; males have larger dorsal and anal fins), water below 50°F (10°C), or parents that are old or underfed.

My fry keep dying one after another.

Almost always starvation or fouled water. Start feeding by day 3 with powder food or green water, and double the container's water volume if deaths continue.

Medaka · Japanese Rice Fish

Medaka Eggs: How to Hatch Japanese Rice Fish Eggs and Raise the Fry

By Motoki Totsugi, founder of Tokyo Aqua Garden, writing from Tokyo · Updated September 2026

Close-up of medaka eggs with the embryos' dark eyes clearly visible — Medaka eggs a few days before hatching. Once the eyes show, switch to dechlorinated water.

Once the water warms past 68°F (20°C), medaka (the Japanese rice fish, Oryzias latipes) start laying eggs almost every day. In Japan, children raise them on apartment balconies every summer, and breeders have created over 450 named varieties. It is the most-read article our team has published in Japanese, and this is the English edition.

The eggs hatch in roughly 10–14 days. How many make it depends mostly on two things: keeping fungus off the eggs, and having tiny food ready before the fry run out of yolk. Below is the routine our staff follow in Tokyo.

The 30-Day Timeline: From Egg to Stable Fry

The days below assume water around 68°F (20°C). At 77°F (25°C), hatching comes about three days sooner.

Days	What happens	What you do
1–3	Eggs laid (females carry them, then attach them to plants)	Collect eggs into a separate container. Plain tap water is fine at this stage (see below)
4–7	Infertile eggs turn cloudy white	Remove white eggs daily with a pipette; change water daily
8–12	Dark eyes become visible inside the eggs	Hatching is near. Switch to dechlorinated water now
13–14	Hatching (usually in the morning, rarely all at once)	Do not feed yet; fry live off the yolk sac
15+	Yolk sac used up; the risk of starvation climbs fast	Feed powder fry food, infusoria, or green water several times daily

Setting Up a Hatching Container

Illustration by Satoko Nakajima — A small container is all the hatching gear you need.

A food-storage container or a large deli cup is enough. Allow about 2 cups (500 ml) of water per clutch of eggs, and pick one with a wide opening so oxygen can dissolve. At this size you need no filter and no airstone. For bigger batches (half a gallon and up), a gentle air stone raises hatch rates by keeping the water moving.

The smaller the container, the faster the water fouls, so change it every day without skipping.

Medaka eggs collected in a small hatching container, viewed from above — A simple hatching container, enough for one clutch of eggs.

The Japanese Tap Water Method, and One US Warning

Almost every medaka breeder in Japan keeps the eggs in plain, untreated tap water for the first week, changed daily. The chlorine that would harm fish protects the eggs by holding back fungus and bacteria, and the eggshell shields the embryo.

⚠️ US water is different, so check before trying this. Many US municipalities disinfect with chloramine instead of chlorine. Chloramine lasts longer in the water and is harsher. The daily-change routine still works where the water is chlorinated, but if your utility uses chloramine (your water quality report will say), skip this method and use methylene blue instead. In every case, switch to dechlorinated water once you see eyes in the eggs. Chlorine kills newly hatched fry.

Methylene Blue for Fungus

The standard treatment for egg fungus, in Japan and the US alike, is methylene blue (Kordon and API sell it in the US). Use a weak dilution. It stains infertile eggs blue, which makes them easy to spot and remove, while healthy eggs stay clear. Light breaks it down, so re-dose after each water change. As with the tap water method, move the eggs to clean dechlorinated water just before they hatch.

Light and Temperature

Eggs develop best with 13–14 hours of light a day. A windowsill works, or any aquarium or plant LED on a timer. Aim for water around 77°F (25°C).

A reader once asked in the comments on our Japanese edition whether 28°C (82°F) water was a problem. We replied that medaka can be raised without trouble at 28°C, and that hatching and raising them at high temperatures sometimes produces daruma medaka.

Japanese breeders time hatching with the degree-day rule (積算温度, sekisan-ondo): eggs hatch once the accumulated temperature reaches about 250°C·days. At common temperatures that works out to:

Water temperature	Days to hatch
77°F (25°C)	~10 days
72°F (22°C)	~11–12 days
68°F (20°C)	~13 days

Small containers in direct sun overheat quickly. Keep a thermometer in the container itself, not just in the room.

Why Eggs Get Fungus (and How to Stop It)

White Eggs Are Infertile — Remove Them Daily

A few days after laying, infertile eggs turn cloudy white. They will never hatch, and fungus starts on them and spreads to the healthy eggs. Pull them out as soon as you see them. Fertile eggs are firmer than they look, and a pipette or even your fingertips won't hurt them.

Break Up Egg Clumps

Freshly laid eggs carry sticky threads (fuchakushi (付着糸) , "attachment threads") that bind them into clumps. Water stagnates between clumped eggs, and they mold faster. Gently separate clumps into single eggs; some breeders roll eggs on gauze to strip the threads off entirely. If you're new to this, don't force it. Even eggs left in clumps mostly hatch if you change the water daily.

With the routine above (daily water changes, pulling white eggs, 13–14 hours of light at 68–77°F), our staff consistently see 70–90% hatch rates. Eggs left alone in the parent tank manage only 30–50%, because the parents eat them or fungus gets them. None of it costs money.

The First Week After Hatching

A newly hatched transparent medaka fry near the water surface — A newly hatched medaka fry, still living off its yolk sac. It has 2–3 days before it needs food.

This page covers the first week. The rest of the first month, including when the fry can safely join the adults, is in Raising Medaka Fry: Getting Them Past the First Month.

Protect the Fry from Filters and from Their Parents

Move the fry, scooped up with some of their water, to a roomy grow-out container. Skip the filter at first: newly hatched medaka can't swim against any current, and a standard filter will suck them in. A gentle sponge filter is safe once they've grown a little. And don't put fry back with the adults yet. Medaka parents eat their young without hesitation.

Feeding the Fry

Fry can't fit adult food in their mouths. From day 3, feed several times a day with any of these:

Food	Notes
Powdered fry food	Easiest. Hikari First Bites is the US-available standard (Hikari is a Japanese brand we also use in the tanks we maintain)
Infusoria / paramecium culture	What Japanese breeders culture at home from rice-rinse water or yeast. Live food gives the fastest growth
Green water	Water tinted green with phytoplankton, which works as a self-serve buffet. It grows on its own in a sunny container with a little fish waste

Aim for a container where food is always available. Fry can't go long between meals.

A green water culture beneath floating plants in an outdoor container — Green water in an outdoor container, the traditional Japanese way to feed medaka fry.

When Can Fry Rejoin the Parents?

When they no longer fit in an adult's mouth: about 0.8 inches (2 cm), or two-thirds of the parents' size. Any earlier and they're on the menu.

FAQ

Q. My eggs turned completely white. Will they hatch?: A. No. White eggs are infertile or have stopped developing. Remove them quickly so fungus doesn't spread to the healthy eggs.
Q. How many eggs does a medaka lay?: A. 10–30 per spawning, and they spawn almost daily in season (early summer to fall). One female can produce several hundred eggs in a season.
Q. The parents keep eating the eggs. How do I stop them?: A. You can't train them, so separate the eggs instead. Collect eggs in the morning (spawning peaks at dawn), or use a spawning mop you can lift out whole.
Q. None of my eggs are ever fertile. What's wrong?: A. Usually one of three things: no functioning male (check the fins; males have larger dorsal and anal fins), water below 50°F (10°C), or parents that are old or underfed.
Q. My fry keep dying one after another.: A. Almost always starvation or fouled water. Start feeding by day 3 with powder food or green water, and double the container's water volume if deaths continue.

Next in this series

Raising Medaka Fry: Getting Them Past the First MonthWhat the next four weeks look like.

About Tokyo Aqua Garden

We are a professional aquarium design & maintenance company based in Tokyo, Japan. Our aquarists install and maintain aquariums for offices, clinics and homes across Tokyo, and we have published more than 3,600 aquarium care articles in Japanese. Hatching medaka eggs is the most-read thing we have ever published in Japanese, because it is the moment a keeper becomes a breeder.

Originally published in Japanese on t-aquagarden.com, where it is our most-read article. This English version has been adapted for international readers. Units, water-treatment differences (chlorine vs chloramine) and products have been localized for the US.