Medaka Egg Hatching Guide — From Egg Collection to Fry Care

Medaka breeding is a hobby that even beginners can enjoy with relative ease, but whether or not you know how to manage the eggs makes a huge difference in hatch rates. In order to avoid wasting the eggs that are born, we will explain the steps from collecting eggs to hatching and initial care of fry.

Spawning Conditions and Egg Collection Timing

Medaka begin spawning behavior when water temperature reaches 20°C or higher and day length exceeds 13 hours. In outdoor breeding, the spawning season runs from late April through around September. In indoor breeding, spawning can occur year-round if conditions are set using heaters and lighting. Females typically spawn in the early morning, and can be observed swimming with eggs hanging from their bellies. Laid eggs attach to aquatic plants or spawning beds. It is ideal to collect eggs daily, as leaving eggs in the same container as parent fish risks them being eaten. Spawning beds have traditionally used the roots of water sprite or shredded palm fiber, but in recent years, commercially available sponge spawning beds have become popular for their convenience. Since egg collection is completed simply by moving the spawning bed to another container, there is no need to touch the eggs directly with your hands.

Egg Management Method

Collected eggs should be managed in a separate container from parent fish. A small plastic case or container is sufficient. Use tap water directly without dechlorination. The trace amount of chlorine in tap water helps prevent mold on the eggs. The amount of water should be just enough to submerge the eggs, and a water depth of around 5 cm is fine. The optimal water temperature is 25–28°C, and hatching will occur in approximately 10 days at this temperature range. The accumulated temperature is approximately 250°C days (water temperature × number of days) as a guideline. For example, at 25°C water temperature it takes about 10 days, and at 28°C about 9 days to hatch. During egg management, change the water daily and maintain a clean environment. When observing eggs, fertilized eggs are translucent and black eyes will become visible inside. White, cloudy eggs are unfertilized or moldy eggs, so remove them as soon as you find them.

Mold Prevention and Tips for Increasing Hatch Rate

The egg's biggest enemy is mold (water mold). Mold develops from unfertilized eggs and spreads to surrounding healthy fertilized eggs, hindering hatching. There are three basic mold prevention strategies. First, remove unfertilized eggs early. Observe eggs daily and remove any white, cloudy eggs with tweezers or a pipette. Second, separate the eggs from each other. Freshly laid eggs are connected by thread-like filaments, but gently loosening them with your fingertips to separate them one by one prevents mold from spreading to adjacent eggs. Eggs are surprisingly durable and won't easily break if pinched. Third, there is a method of managing eggs in water with diluted methylene blue. Methylene blue has the effect of inhibiting water mold development and can be obtained as a drug for ornamental fish. A concentration just light enough to turn pale blue is sufficient. By combining these strategies, hatch rates can be significantly improved.

Fry Care Immediately After Hatching

Fry that have just hatched (called "needle fish") are extremely small fish with a body length of around 2mm, and the first two weeks is a critical period that determines survival rates. For 2–3 days after hatching, fry sustain themselves on nutrients from the yolk sac, so feeding is not necessary. Once the yolk sac is depleted, feeding begins, but normal adult food pellets are too large to eat. Use powdered fry food or crush adult food with your fingers into powder before feeding. If raised in green water (water where plant plankton have proliferated), fry can constantly eat fine food, improving survival rates. Culturing and feeding paramecia is also effective. During water changes, young fry risk being sucked up, so limit changes to removing a small amount of bottom debris with a pipette.

Getting Breeding Medaka from Bri-Choku

To enjoy medaka breeding, obtaining quality breeding stock is important. Through Bri-Choku, you can purchase breeding pairs or trios (one male and two females) from breeders with proven breeding records, and receive advice on precautions and selection points for breeding that particular variety. Find a breeder you can consult with about everything from egg management to fry rearing, and take the first step into medaka breeding.