How to Build a Medaka Biotope — Recreating Natural Outdoor Habitats

Medaka biotopes are a breeding style that recreates a natural aquatic environment in a small container. Without using pumps or filters, the ecosystem maintains water quality through aquatic plants and microorganisms—making it the breeding environment closest to medaka's natural ecology. Its visual beauty and the ability to enjoy medaka while experiencing seasonal changes are its greatest appeals.

Basic Concept of Biotopes

A biotope is a German word meaning "habitat for living organisms," referring to a small artificial ecosystem. In a medaka biotope, medaka, aquatic plants, microorganisms, and snails maintain a balanced environment through their interactions.

Medaka waste and uneaten food are broken down by bacteria, and the nutrients are absorbed by aquatic plants. Plants produce oxygen through photosynthesis, replenishing the oxygen needed for medaka respiration. When this cycle works well, water quality remains stable without a filter.

Choosing a Container

Biotope containers come in various options.

Water lily basin (suiren bachi): The standard biotope container. The ceramic texture creates a Japanese aesthetic and offers excellent heat retention. Choose one with a diameter of 40cm or larger and a capacity of 20 liters or more.

Plastic trough (toro bune): A plastic container used in construction work, offering affordability and large capacity. Large sizes of 40 or 60 liters allow breeding of many medaka and are suitable for reproduction. While simple in appearance, placing bricks or wood panels around it improves the landscape.

Styrofoam container: Extremely high insulation protects water temperature from summer heat and winter freezing. Black styrofoam containers make medaka body color appear darker, making them popular with breeders.

Wooden barrels or wine barrels: Atmospheric containers that suit Western-style gardens. However, choose ones coated with food-safe waterproof coating to prevent harmful substances from dissolving into the water.

The key consideration when choosing a container is water volume. Secure at least 10 liters minimum, ideally 20 liters or more. Greater water volume results in smaller fluctuations in temperature and water quality, creating a more stable environment for medaka.

Substrate Selection and Placement

Substrate is an important element as it serves as a habitat for water-purifying bacteria.

Akadama soil: The standard biotope substrate. Its porous nature is excellent for bacterial colonization, and it creates slightly acidic water. Hard akadama soil is more durable and doesn't crumble easily. Small or medium grain sizes are easiest to handle.

Oososhina sand (fine grade): A permanent substrate with minimal water quality impact. While somewhat heavy and suitable for planting aquatic plants, it offers less surface area for bacterial colonization than akadama soil.

Arakita soil: Natural paddy field soil closest to medaka's natural habitat. It promotes growth of microorganisms like Daphnia, which serve as initial food for fry, though water tends to become cloudy.

Substrate should be layered 3–5cm thick. Creating an incline with the back thicker and front thinner makes it easier to observe medaka from above.

Aquatic Plant Selection and Arrangement

Choose aquatic plants for biotopes that can withstand strong outdoor sunlight and temperature fluctuations.

Floating plants: Water hyssop (Eichhornia crassipes) is a representative floating plant for biotopes. It provides natural shade in summer and its roots serve as a spawning bed for medaka. Combining smaller floating plants like Amazon frogbit and salvinia works well. However, if floating plants cover too much of the water surface, light won't reach the bottom, so keep about two-thirds of the surface clear.

Submerged plants: Anacharis (Egeria densa) is a standard aquatic plant that's hardy, grows quickly, and has excellent water purification ability. Hornwort can also be grown just by floating it and provides hiding places for medaka.

Emergent plants: Plants with roots in water and stems or leaves extending above the surface. Water lilies are beautiful when flowering and their leaves shade medaka. Small emergent plants like ribbon arrowhead and aquatic mushroom add color.

Marginal plants: Water-margin plants like Japanese white-eye sedge and miniature papyrus, placed in small pots at the container's edge, add height and natural appeal to the biotope.

Introducing Organisms

Adding organisms that coexist with medaka creates better balance in the biotope.

Amano shrimp (Caridina japonica): Cleans algae and leftover food. Has good compatibility with medaka and strong reproduction rates. Include 3–5 for water purification contribution.

Snails (Japanese trapdoor snails): Eat algae on walls and suspended particles, clearing the water. Their filter-feeding ability can even clear green water (phytoplankton blooms).

Loach: Excellent bottom cleaners, but larger individuals can cause stress to medaka. Small species like zebra loach are recommended.

The medaka stocking density guideline is one fish per liter, though biotopes are ideally stocked slightly below this rate. A 20-liter container suits 10–15 medaka.

Seasonal Management

Spring (March–May): As water temperature rises, medaka activity increases. Remove winter debris with a 30% water change. Once temperature exceeds 18°C, breeding begins—provide spawning materials.

Summer (June–August): Direct sunlight raising water temperature is the main challenge. Use reed screens or shade nets to reduce sun exposure, keeping water below 35°C. Floating plants naturally shade the water, so slightly more is fine this season. Change water frequently to compensate for rapid evaporation.

Autumn (September–November): Build up medaka's energy reserves for winter. Provide quality food generously. Prevent fallen leaves from entering the water, and reduce feeding as temperature drops below 15°C. Remove cold-sensitive plants like water hyssop indoors or maintain backup stock.

Winter (December–February): Below 5°C, medaka become inactive on the bottom and stop eating. Stop feeding entirely. Medaka can overwinter if the bottom doesn't freeze even if the surface freezes. Insulated containers (styrofoam) are advantageous for overwintering.

Finding Hardy Medaka for Outdoor Cultivation at Br-Choku

To obtain hardy medaka that thrive in biotopes, consulting with Br-Choku breeders is optimal. Medaka raised in outdoor cultivation are highly adaptable to environmental changes and integrate smoothly into biotopes. You can receive direct breeder advice on outdoor cultivation suitability for each variety.