Zebra Danio

About the species

The zebra danio (Danio rerio) is one of the few aquarium fish known well beyond the hobby – it's one of the most widely used model organisms in genetic and developmental research, and specimens have even been sent into space. In the tank it's just as undemanding as in the lab: it tolerates a wide range of temperature and water chemistry, but it's an active swimmer that needs tank length above all, not just volume.

Tank length matters more than depth

A footprint of at least 90 × 30 cm (roughly 80 litres) is recommended even for a small shoal. Zebra danios are constantly on the move and look restless in short, tall tanks regardless of otherwise adequate volume.

Water that asks for nothing special

A temperature of 18–24 °C, pH of 6.0–7.5, and general hardness of 5–19 °dH cover most tap water; carbonate hardness of roughly 4–12 °dH is fine. Unlike mollies or swordtails, they don't need specifically hard or alkaline water, so a heater is often unnecessary in cooler rooms.

A shoal of at least eight to ten

This is a shoaling species – in groups smaller than eight, zebra danios look nervous and duller. In a larger shoal, males compete for female attention, which brings out their best colour.

Décor and choosing tankmates

Colours show best in a densely planted tank with a dark substrate and floating plants or roots that diffuse the light. Zebra danios are very peaceful and combine well with many popular aquarium fish – smaller cyprinids, tetras, livebearers, rainbowfish, anabantoids, catfish and loaches – though tankmate size should always be checked against their own small size.

Beyond the wild-type form: bred colour variants

Alongside the standard striped form, bred variants are available, including the spotted 'leopard danio' and a long-finned form, both cared for identically to the standard fish. The leopard variant's origin was long a mystery but is now known to be a spontaneous mutation that also occurs in the wild.

Origin and natural habitat

The species was described from the Kosi River, a Ganges tributary in the Indian state of Uttar Pradesh. A 2007 field survey recorded it at around twenty sites in northeastern India, often in slow or nearly still water over silty substrates with dense submerged vegetation – including seasonally flooded rice paddies, which researchers hypothesise it moves into during the monsoon to spawn.

Not ideal company for slow, long-finned fish

Zebra danios are generally very peaceful toward most tankmates, but in too small a shoal or with too few of their own kind, they can occasionally nip the fins of slower, long-finned species such as some bettas. A large enough shoal and enough space reduce this risk.

The 'GloFish' variant isn't legal in the EU

Genetically modified, fluorescent zebra danios (marketed as GloFish), originally developed to detect environmental toxins, are illegal to buy, sell or own within the European Union. They aren't a legal option in Slovenia, even though they're popular in some other countries such as the United States.

The risk with genetically weakened lines

Mass production has left part of the commercial supply genetically weakened and susceptible to so-called neon tetra disease (NTD) – an untreatable parasitic illness marked by white patches, rapid breathing and an affected fish separating from the shoal. There's no medication; the only response is removing affected fish.

Breeding

Zebra danios are egg-scattering spawners with no parental care and one of the easiest species to breed under controlled conditions in the aquarium – eggs hatch in 24 to 36 hours, and fry are free-swimming within a few days.

Pros & cons