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

Danio rerio was described by Francis Hamilton in 1822 from his survey of the fishes of the Ganges. It is a small danionin of South Asia, native to the southern Himalayan drainages from the Sutlej basin on the Pakistan-India border east to Arunachal Pradesh, and common across much of India, Bangladesh, and lowland Nepal. It lives in shallow, slow or still water: secondary river channels, irrigation canals, ditches, wetlands, and paddy fields. Wild fish meet a wide temperature swing over the year, from around 6 C in winter to above 38 C in summer, part of why the species is so hardy in tanks. The body carries five dark horizontal stripes that run back into the tail; males are slimmer with gold between the blue stripes, while females are fuller-bodied with a whitish belly and silver tones. Wild adults usually run 1.8 to 3.7 cm, with aquarium fish reaching about 5 cm. Hobby strains include a long-finned form, a golden form, an albino, the spotted 'leopard' (once described as Brachydanio frankei and now known to be a spontaneous mutation of the striped fish), and the fluorescent transgenic GloFish, of which the zebra danio was the first sold. Some taxonomists place the species in the genus Brachydanio, though Danio rerio remains the dominant usage. Far beyond the aquarium, this is one of the most important vertebrate model organisms in biology and medicine, valued for its transparent embryos, fast development, and fully sequenced genome. The IUCN lists it as Least Concern. Released and escaped fish, including GloFish, have established in parts of the United States, Colombia, and Brazil.

Breeding

One of the simplest aquarium fish to breed, and often a beginner's first project. Condition a group on rich foods for a week, then move two or three males with one or two well-fed females into a shallow tank floored with marbles or a mesh grid so the scattered eggs drop out of the adults' reach. Spawning fires at first light and is over in minutes, the group dashing about and releasing several hundred eggs, sometimes four or five hundred from a productive group. Pull the adults straight afterward, since they hunt down and eat the eggs. Eggs hatch in roughly two to three days, the fry are free-swimming a couple of days after that, and they take baby brine shrimp right away. Growth is quick: juvenile striping shows within a few weeks and the fish near adult size in two to three months. That easy, predictable spawning is one reason the species became a laboratory staple. In a warm outdoor pond a single batch can yield thousands of fry.

Common problems

Hardy almost to a fault, with little serious disease in an established tank. Ich shows up mainly in newly bought, stressed fish and clears with standard treatment. The notable concern is mycobacteriosis, or fish TB, a chronic Mycobacterium infection that appears in older danios and is the second most common disease in laboratory zebrafish. It shows as spinal curvature, wasting, and lethargy, builds over months, and is difficult to cure, so badly affected fish are usually euthanized humanely. Jumping is the other constant: these fish leap when startled, while chasing each other, and seemingly at random, so a gap-free lid is essential. Reports of shorter lifespans in some GloFish strains compared with wild-type fish are 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

• Biotope aquariums: matching fish to their natural habitat
• The schooling number matters more than you think