Zebra danio

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: plant safe.

Feeding

Eats everything. Flake food, micro pellets, frozen bloodworm, frozen brine shrimp, frozen daphnia, live food, crushed peas. Feeds at the surface and in the upper water column. Fast eaters that grab food before it sinks, which means slower bottom-feeders in the same tank need targeted sinking food. Feed twice daily. Not picky about quality or type. Live fruit flies and wingless drosophila are a natural treat since wild danios eat insects that fall on the water surface.

Compatibility

• Active, fast, and hardy. Zebra danios occupy the top third of the tank and swim in constant rapid loops. They set the energy level of the tank; timid species may be overwhelmed by the activity.
• Minor fin-nipping tendency, especially in small groups. Groups of 8+ keep the nipping mostly within the school. Long-finned tankmates (bettas, fancy guppies) are at risk in smaller groups.
• Good dither fish for shy species. The constant surface activity signals "safe" to bottom-dwellers and cichlids that otherwise hide. A school of danios can draw out a pair of kribs or apistos that would otherwise stay in their caves.
• One of the most commonly used fish in scientific research (as the zebrafish, Danio rerio). Its genome is fully mapped and it's the vertebrate equivalent of Drosophila for genetics research.

Origin and habitat

Native to South Asia: India, Pakistan, Bangladesh, Nepal, and Bhutan; concentrated in the Ganges and Brahmaputra river basins. Range extends from the Sutlej river basin on the Pakistan-India border to Arunachal Pradesh in northeast India, with scattered populations in the Western and Eastern Ghats. Claims of occurrence in Myanmar are based on pre-1930 records and most likely refer to closely related species such as Danio quagga and D. kyathit. Described by Hamilton in 1822 from the Kosi River. Found in shallow, low-flow habitats with sandy substrates: secondary channels, irrigation canals, ditches, wetlands, and paddy fields. Adapted to a remarkably broad temperature range in the wild, from about 6°C in winter to 38°C in summer. Body has five prominent dark horizontal stripes with lighter interstripes; males show gold between blue stripes, females silver. Wild individuals typically reach 1.8–3.7 cm; maximum recorded about 6.4 cm. Multiple captive-bred variants exist: long-fin, leopard (spotted), albino, and GloFish (fluorescent transgenic forms). Beyond the aquarium trade, D. rerio is one of the most important vertebrate model organisms in biomedical research, used extensively in drug development, genetics, and developmental biology due to its transparent embryos and rapid development. Group spawners triggered by monsoon conditions in the wild. Introduced populations exist in the US, Colombia, and elsewhere.

Breeding

One of the easiest aquarium fish to breed, and frequently the first breeding project for new hobbyists. Condition a group with frozen food for a week. Move 2-3 males and 1-2 plump females to a shallow breeding tank with marbles on the bottom (the eggs fall between the marbles where adults can't reach them) or a mesh screen separator. Spawning is triggered by the first light of day and happens within minutes. The group dashes around scattering hundreds of eggs. Remove adults immediately; they eat every egg they can find. Eggs hatch in 48-72 hours. Fry are free-swimming in another 2 days and can eat baby brine shrimp immediately. Growth is fast; juvenile striping appears within 2-3 weeks and the fish reach adult size in 2-3 months. The zebrafish genome project used this species specifically because it's so easy to breed in controlled conditions. Outdoor pond spawning in warm climates produces thousands of fry per batch.

Common problems

Hardy to the point of being boring for experienced keepers. Significant disease is uncommon in established tanks. Ich appears in newly purchased, stressed fish and responds to standard treatment. Mycobacterial infection (fish TB) can occur in older zebra danios, showing as spinal curvature, wasting, and lethargy. It's not treatable and affected fish should be euthanized. This disease is well-documented in zebrafish because of the species' use in laboratory research; it's a chronic infection that takes months to progress. Jumping is common: zebra danios jump when startled, when chasing each other during spawning behavior, and sometimes apparently for fun. A lid with no gaps is mandatory. The GloFish variants occasionally show reduced lifespan compared to wild-type, but the data is anecdotal.

Outdoor pond suitability

Climate

subtropical

USDA zones

8–12 (winter low around -12°C or warmer)

Outdoor pond at least 60 cm deep for thermal mass. Local frost depth and surface freezing matter.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size