Medaka Breeding Guide: From Spawning to Fry Rearing

Medaka (Japanese rice fish) breeding is relatively easy, and with proper conditions, spawning can continue from spring through fall. Raising fry from eggs to adulthood is one of the most rewarding aspects of medaka keeping.

Conditions Required for Spawning

Water Temperature Temperature is crucial for triggering medaka spawning. - Optimal spawning temperature: 18-30°C (ideal: 25-26°C) - Spawning activity declines below 18°C - For outdoor keeping, the main spawning season is April-October

Day Length At least 13 hours of daylight (or artificial lighting) triggers spawning. Indoor keeping with supplemental lighting can enable year-round breeding.

Nutrition For prolific spawning, feed parent fish high-quality food 2-3 times daily. Specialized high-protein, high-calorie spawning diets are commercially available.

Spawning Beds and Egg Collection

Types of Spawning Beds - Commercial spawning mops (long-fiber sponge type) - Palm fiber (coconut tree bark) - Live plants (Water Hyacinth, Hornwort, etc.) - DIY yarn spawning beds

Females deposit eggs onto the spawning bed once installed.

Egg Collection Method 1. Check the spawning bed each morning for eggs 2. Gently remove eggs from the bed with your fingers (eggs are robust enough to handle) 3. Transfer eggs to a separate container (plastic case or small tank) 4. Return the spawning bed to the main tank

Egg Management - Eggs must be kept in a separate container, as adults and other fish will eat them - At 25°C, eggs hatch in approximately 10 days (lower temperatures extend incubation time) - Keep water clean with daily water changes (or water circulation) - Remove white unfertilized eggs promptly

Fry Care

Post-Hatching Management Newly hatched fry (called "hariko" in Japanese) are only 0.5-1mm in size and survive on yolk sac nutrition for 2-3 days without feeding.

Fry Food Options - PSB (Photosynthetic Bacteria): Highly nutritious, small enough for newly hatched fry - Paramecium: Live microorganisms with excellent digestibility. Can be home-cultured with culture kits - Commercial Powdered Fry Food: Finely ground for easy consumption - Green Water: Rich in phytoplankton that serves as a nutritional source for fry

Feed small amounts 3-5 times daily. Avoid overfeeding, as leftovers quickly degrade water quality.

Container Progression by Growth Stage

| Growth Stage | Size Guide | Recommended Container | |-------------|-----------|----------------------| | Newly hatched | 0.5-3mm | Small plastic case (5-10L) | | Fry | 3-10mm | Medium container (10-20L) | | Juvenile | 10-15mm | Same as adult container | | Adult | 15mm+ | Can cohabitate with adults |

It's safe to add fry to adult containers once they reach about half the adult body length (15-20mm).

Keep Breeding Records

Recording which breed produced which fry makes strain management much easier. Label containers with masking tape, noting the date and parent fish information.