Care
The Medaka is a selectively bred form of the Japanese Rice Fish (O. latipes). The original wild form is silvery-grey, and is known from freshwater ponds, canals, marshes, paddy fields, slow-moving streams, and brackish coastal waters.
This species is an excellent choice for the unheated aquarium; it is hardy, peaceful, stays small, and reproduces readily via a fascinating method (see breeding section below). The tank should be mature and have areas of driftwood tangles and dense planting, ideally tall species that grow close to the surface, plus some floating plants. In the wild, these sociable rice fish feed in large groups near to the water's surface, consuming insect larvae and tiny crustaceans; therefore they are best maintained in good sized groups (6 specimens minimum, 10+ is better). Filtration should be efficient, but the flow should not be too vigorous as these fish prefer calmer waters. O. latipes is best maintained in a species-only tank, but they can be kept as part of a community of sub-tropical fish of similar size and temperament. Ensure that the tank has tight fitting coverslides as these fish can jump. In Japan these fish are widely kept in outdoor containers but this style of keeping is currently prohibited by UK legislation.
May also be seen on sale as Yellow princess ricefish, Japanese Medaka, or labelled by the name of the many fancy strains that are occasionally seen.
Feeding
Small frozen foods such as bloodworm, white mosquito larvae, daphnia, baby brineshrimp (Artemia nauplii), vitamin-enriched brineshrimp etc. Will also take flake/micropellets, but small meaty frozen foods should make up the bulk of the diet.
Breeding
These fish are external brooders and are fairly easy to breed. Females actually carry eggs for some time before spawning occurs. The oocytes mature at night, followed by ovulation at dawn, with spawning typically occurring when the first rays of sunlight hit the aquarium glass; the male fish intensifying in colour and attempting to entice females whilst defending small transient territories. The female will expel a cluster of relatively large, adhesive eggs, which the male fertilises simultaneously. However, instead of depositing the eggs immediately, they stay attached to the genital opening of the female for a time before being set down singly or in small clumps amongst fine-leaved plants (or spawning mops). The eggs will hatch within 10-14 days, and is very much temperature dependent. Adult fish usually ignore the eggs, but they will predate on the fry, so unless the tank is densely planted to offer shelter to the young, it is best to either move the adults to another aquarium or else move the eggs to a small tank filled with water from the main tank. Once free swimming, the fry can be offered baby brineshrimp (Artemia nauplii), microworm, and powdered fry foods. Larger sized fry will predate on smaller sized fry, so do keep fry of different ages separate until there is less of a discrepancy in size.
