Zebra danio

Quick facts

Adult size

5 cm

Lifespan

can live up to 5 years; captive average is 3-4

Tank zone

mid

Temperament

peaceful

Difficulty

beginner

Schooling

recommended 6+ (critical minimum 5, thrives at 8+)

Water parameters

Temperature

18–26°C

pH

6.0 to 8.0

Hardness

5 to 20 dGH

Tank requirements

Minimum volume

75 L

Minimum length

75 cm

Flow

moderate

Lighting

moderate

Substrate

any

Open swimming room

needed

Lid

required - jumper

Feeding

Diet: omnivore, feeds primarily at the all.

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.

Habitat

Native to South Asia: India, Nepal, Bangladesh, Myanmar, and Pakistan. Found in a wide range of habitats, from fast-flowing mountain streams to slow rice paddies and drainage ditches. This ecological flexibility is reflected in the species' extreme hardiness in the aquarium; it tolerates temperatures from 18 to 28°C, pH from 6.0 to 8.0, and hardness from 5 to 20 dGH. Originally described by Hamilton in 1822. The species has been in the aquarium hobby since the early 1900s and remains one of the best-selling freshwater fish worldwide. Body is elongated and torpedo-shaped with horizontal blue-black and silver-gold stripes running from behind the gill to the tail. Multiple variants have been bred: long-fin, leopard (spotted instead of striped), albino, and GloFish (fluorescent, genetically modified). The GloFish variants were the first commercially available genetically modified pet and remain controversial in some markets. Adult size is 5–6 cm. Males are slimmer; females are rounder when full of eggs. In the wild, danios are seasonal breeders triggered by monsoon rains; in the tank, they'll breed year-round.

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

This species is suited to outdoor ponds, not just indoor aquariums.

Climate classification

subtropical

Outdoor pond zones (USDA)

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

Outside the zone range, this species can still be kept indoors. Within the zone, an outdoor pond at least 60 cm deep usually has enough thermal mass to overwinter the species, though local frost depth and surface freezing matter.

Bioload

Bioload coefficient: 1.6 (5 cm active swimmer; pulled down from formula (~2.3) to align with hobby consensus of 1.5-1.7x neon).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

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