How Many Medaka Can You Keep?

The quick answer

For a first group, plan on roughly one adult medaka per two gallons of water. In a standard ten gallon tank that means five or six fish, which leaves comfortable room and keeps maintenance easy. Adult medaka pass an inch and keep filling out, so the old one-per-gallon shortcut crowds a tank faster than it looks. Medaka are shoaling fish and are visibly more relaxed in a group, so six is a better starting number than three.

Below is what that works out to in the containers people actually use.

"Pushing it" assumes established water, floating plant cover, and a keeper who is paying attention. Start at the comfortable number. *I would not start with a 5 gallon at all: 10 gallons is my recommended minimum, and everything about keeping medaka gets easier with more water.

Why surface area beats gallons

Gallons are a convenient shorthand, but what really limits a medaka population is surface area, the amount of water touching air. That is where oxygen enters and where waste gases leave, and medaka spend most of their time in the top few inches anyway. It is why a wide, shallow tub holds far more fish than a tall, narrow vase of the same volume, and why the traditional Japanese medaka container is a broad, shallow bowl rather than a deep one.

So when you are choosing between two containers, take the wider one. A shallow 20 gallon stock tank is a better medaka home than a tall 20 gallon aquarium, even though the label says the same number.

What raises the ceiling

Three things let a container hold more fish comfortably. Floating plants do the heaviest lifting: they pull nitrogen out of the water continuously and shade the surface, which is why every live order I send includes them free. Age matters too, since an established container with mature biological filtration processes waste far better than one filled last week. And gentle water movement, from a sponge filter or a small air stone, keeps oxygen levels even throughout rather than only at the surface.

What does not raise the ceiling is feeding less. Underfeeding a crowded container produces thin fish, not clean water.

Do not forget the fry

This is the part that catches new keepers out. A stocked medaka container is not a fixed number of fish, it is a starting number. In warm months a healthy female lays most days, and if there are floating plants or a spawning mop in there, some of those eggs will hatch where they land. A tub stocked at a sensible eight can be holding thirty by August.

That is not a problem, it is the fun part, but it means you should stock the adults with room to spare and have a plan: a second container, a friend who wants some, or simply leaving the eggs where the adults will thin them naturally. My breeding guide covers how to take control of that cycle deliberately.

Signs you have too many

Fish tell you before test kits do. Watch for medaka hanging at the surface gulping when the water is warm, which points to low oxygen. Watch for growth stalling in younger fish, or for the water going cloudy and staying that way. Any of those means fewer fish, more surface area, or more plants, and the quickest fix is usually to move a handful into a second container rather than to buy equipment.