How to Tell If Medaka Eggs Are Fertilized

Written by Chris, the breeder at Firefly Medaka

The short answer: a fertile medaka egg stays clear, and an infertile one turns chalky, opaque white, usually within a day or two. You do not need a microscope or any equipment to tell them apart, just a little light and a bit of patience.

The quick test

Hold the eggs up to a light source, a window or a lamp behind them, and look.

Clear and slightly amber, like a tiny glass bead. That is a good egg. You may see fine detail inside it. Fertile medaka eggs are firm enough to handle gently, which is unusual among fish eggs and part of what makes this species so easy to breed.

Opaque, chalky white, like a grain of rice. That one is finished. It was either never fertilized or stopped developing, and it will not recover. Cloudy white is the single clearest signal you will get, and it does not require any judgement.

Do not judge on day one. A freshly laid egg has not had time to show anything. Give it a day or two, then sort. Discarding early is how people throw away eggs that were perfectly fine.

What you see as it develops

A fertile egg does not just sit there. Over the days before hatching you can watch it change, which is genuinely one of the more rewarding things about keeping this species.

Early on it stays clear. Partway through you may see a faint darkening and the beginnings of structure. Then, unmistakably, two dark eyes appear, and once you can see eyes the egg is well along and clearly viable. Near the end you can sometimes see movement inside the shell.

Hatching takes roughly nine to eleven days in the mid to high seventies Fahrenheit and longer in cooler water, because development runs on accumulated warmth rather than a fixed number of days. So if your eggs seem slow, check the temperature before assuming they are dead.

Why the bad ones matter

An infertile egg does not simply sit inert. It grows fungus, and that fungus spreads to healthy eggs sitting against it. One overlooked white egg in a tight cluster can take several good ones with it.

So the practical reason to sort is not tidiness, it is protecting the rest of the batch. Check every day or two and remove anything that has gone opaque. A pipette, a spoon, or clean fingers all work, and the eggs are tougher than they look.

Keeping them in a shallow container with a little water movement also helps, since still water in a warm room is where fungus does best.

If they are all infertile

A whole batch going white points at the parents rather than the eggs. The usual cause is no male in the group, or not an active one. Females will lay unfertilized eggs quite happily on their own, which surprises people, so eggs appearing is not by itself proof you have both sexes.

Males have a longer, larger dorsal fin with a distinct notch, and a longer, more parallel-sided anal fin. A working ratio is two or three females per male. If you are getting eggs but none develop, that is the first thing to check, and my guide to a group that is not producing covers the rest.

What to do with the good ones

Move them somewhere the adults cannot reach, because medaka will eat their own eggs and fry. A separate container of water from the same system is all it takes. A spawning mop makes this easy: females wipe eggs onto the yarn, you lift the whole mop into a hatching container, and drop a spare back in.

From there my fry care guide covers hatching and the first fortnight, which is where nearly all losses actually happen.

Sort them the easy way

Handmade spawning mops

Eggs land on the yarn where you can actually see them, so checking which are clear and which have gone white takes a minute. Lift the whole mop into a hatching container and drop a spare back in.

Shop Spawning Mops

Common questions

How can you tell if medaka eggs are fertilized?

Hold them up to a light. A fertile egg stays clear and slightly amber, like a tiny glass bead, and develops visible dark eyes before hatching. An infertile egg turns opaque, chalky white like a grain of rice, usually within a day or two, and will not recover.

Why are my medaka eggs turning white?

Opaque white means the egg was never fertilized or has stopped developing. It cannot be saved, and it should be removed because it grows fungus that spreads to healthy eggs touching it. A whole batch turning white usually means there is no active male in the group.

How long before medaka eggs show eyes?

Eyes become visible partway through development, ahead of hatching, which takes roughly nine to eleven days in the mid to high seventies Fahrenheit. Cooler water stretches the whole timeline, since development depends on accumulated warmth rather than a set number of days.

Can female medaka lay eggs without a male?

Yes. Females will lay unfertilized eggs on their own, so the presence of eggs does not prove you have both sexes. Those eggs simply turn white and never develop. Males have a longer dorsal fin with a distinct notch and a longer anal fin.