Medaka Ricefish

Minimum footprint is a 38 litre (10 US gallon) tank, roughly 50 x 25 x 30 cm, for a group of six; a longer, shallower tank suits their surface-and-midwater swimming better than a tall one. A tight-fitting lid is essential because ricefish are accomplished jumpers. Use a fine sand or smooth gravel substrate and plant heavily, ideally with floating plants (such as duckweed, frogbit or Salvinia) plus fine-leaved stems and spawning mops or moss, which mimic their native rice-paddy vegetation and give fry cover. Add gentle filtration and some open surface water. Bright light and even a little indirect sunlight bring out their colour and encourage breeding.

Omnivore. Feed a quality micro-pellet or crushed flake as the staple, sized for a small mouth, once or twice daily in amounts eaten within a minute or two. Supplement several times a week with small live or frozen foods such as daphnia, baby brine shrimp, microworms, cyclops and mosquito larvae for colour and breeding condition. Vary the diet and include occasional blanched vegetable matter or spirulina. AVOID: overfeeding (the single biggest killer, it fouls water and causes bloat), untreated tap water containing chlorine or chloramine, oily human or table foods, and any copper-based medications or plant fertilisers if you keep shrimp or snails with them, as copper is toxic to invertebrates.

Freshwater. Aim for about 16 to 24 C, which suits both everyday keeping and breeding. Medaka are unusually hardy for their size: they tolerate roughly 10 to 28 C comfortably and can survive brief extremes from about 4 C up to around 40 C as long as the water does not freeze. Heat is not acutely lethal at typical room temperatures, but SUSTAINED warmth above about 26 to 28 C raises metabolism, lowers dissolved oxygen and shortens lifespan, so a stable, cooler tank is best for longevity. They can be kept unheated indoors as long as the room stays above roughly 15 C. pH 7.0 to 8.0 is ideal (tolerant of 6.5 to 8.5). Hardness GH 6 to 20 and KH 5 to 10 give good buffering. Provide gentle filtration and low to moderate flow, good surface agitation for oxygen, a fully cycled tank with 0 ppm ammonia and nitrite, nitrate kept low with weekly partial water changes, and a standard 8 to 10 hour planted-tank light period. No UVB or basking is needed as this is a fish, not a reptile.

A social, loosely-schooling species that must be kept in a group of at least six, ideally more, and does poorly kept singly. It is peaceful and mixes well with other calm fish and shrimp of similar size and temperament. Sexing is straightforward: males have a larger, more angular dorsal and anal fin (the anal fin is broad and roughly parallel-sided), while females are rounder-bellied and carry eggs in a cluster near the vent after spawning. No pair bonding is required; they breed readily in mixed groups.

Enrichment comes from a naturalistic, planted environment: dense and floating plants to forage and shelter in, gentle current, live foods to hunt, and open water to school and display. A group with a balanced sex ratio will spawn regularly, which is normal behaviour and a sign of good welfare. Morning light triggers courtship displays, so a consistent light cycle and some sunlight keep them active and colourful.