Zebra Danio

Danio rerio is a small cyprinid from the streams and rice paddies of South Asia, and the same species widely used as the "zebrafish" in scientific research. It has horizontal blue and gold stripes and is a tireless, fast swimmer.

Tank & water. Zebra danios reach about 4–5 cm (1.5–2 in) but are so active that a 57 litre (15 US gallon) or longer tank suits them better than the small tanks their size suggests — they need length to swim. They are unusually tolerant, thriving from 18–25 °C (64–77 °F), which makes them one of the few common fish comfortable in unheated rooms, and they accept a broad pH 6.5–7.5.

Temperament. They are peaceful but boisterous shoaling fish that should be kept in groups of at least six. In too small a group or too small a tank their energy can tip into fin-nipping, so give them numbers and space. Their hardiness and activity make them a classic choice for a newly cycled first tank.

Diet & care. Danios are omnivores that eat almost any quality flake or small food and feed mostly in the upper water column, darting up to grab it. A lid is wise, as they are strong, willing jumpers. Long-finned and "GloFish" fluorescent varieties exist; the fluorescent forms are the same species, genetically modified to fluoresce, and have the same care needs as the wild-type fish. As a hardy, adaptable species they are often used to help mature a filter, but this should still be done as a careful fish-in cycle with frequent testing and water changes, not as an excuse to skip cycling. Their toughness makes them forgiving, but they are still living animals owed clean, stable water and enough of their own kind to shoal.