Care guide · Freshwater · fish

Redtail notho

Nothobranchius guentheri

Male Nothobranchius guentheri. Photo: Jokuyken15, CC BY 4.0 (resized proportionally). Photo: Jokuyken15 / Wikimedia Commons · Source · CC BY 4.0Proportional resize only; no crop or colour edit. CC BY 4.0 attribution retained.

The redtail notho (Nothobranchius guentheri) is a colourful annual killifish from Unguja, the main island of Zanzibar, Tanzania. Males have a blue-green body and a striking red tail with a dark edge; females are smaller and more subdued. In temporary pools, eggs survive the dry season in the substrate. Adults therefore live briefly and breeding requires planning. Although familiar in killifish keeping, this is not an automatic beginner community fish. More importantly, the species is assessed as Endangered (EN). Choose traceable captive-bred stock, ask about the line’s provenance and leave wild fish and their habitat undisturbed. Never release eggs, substrate or aquarium water into nature.

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At a glance

Temperature: 22–25 °C
pH: 6.5–7.5
GH: 4–10 °dGH
Aquarium length: At least 45 cm
Minimum aquarium volume: At least 41 L
Adult size: Up to 4.1 cm
Group size: At least 3 individuals
Swimming zone: Middle
Temperament: Territorial

About this species

GBIF and FishBase accept Nothobranchius guentheri (Pfeffer, 1893), family Nothobranchiidae. It is endemic to Unguja. Older combinations include Fundulus guentheri, Adiniops guentheri and Aphyosemion guentheri. The species was historically confused with N. melanospilus, which also occurs on Zanzibar. Costa’s 2017 revision separates them using characters including the male’s red tail with a black rear margin, female traits and scale characters. Seriously Fish gives males to about 41 mm standard length and females about 32 mm; FishBase reports around 3.5 and 3.2 cm SL. Males are larger and more colourful. These differences are no reason to mix unverified lines.

Natural habitat

Nothobranchius guentheri lives in temporary pools and marshes in the coastal lowlands of Unguja, Tanzania. These waters fill during the rainy season and may dry completely in the dry season. The species bridges that period as embryos inside eggs left in muddy substrate. Natural pools have been measured at about 25–29°C, pH 7.2–7.7 and conductivity 140–980 μS/cm; these are field observations, not instructions to keep an aquarium warm or turbid. Several known pools lie in farmland and are drained or converted to rice fields. This pressure underlies its Endangered status. In captivity, do not imitate drying literally: keep water stable and support responsible captive lines rather than allowing a home tank to dry out.

Aquarium and layout

Seriously Fish gives a base of 45 × 30 cm and about 41 litres for one male with two or three females. Volume depends on tank height. Use a dark, soft substrate, dense planting, floating plants and wood for cover, with subdued lighting. A sponge filter with gentle flow suits the fish better than a strong outlet. A secure lid is essential because it jumps. The aquarium should be biologically mature and maintained regularly. Arrange it so every fish can be observed and fed. A breeding tank is temporary and does not replace permanent space for the group. A species tank is preferable, especially when preserving a line or breeding deliberately.

Water conditions

For aquarium maintenance, Seriously Fish gives 22–25°C and pH 6.5–8.0. FishBase reports natural pool values of 22–25°C, pH 6–7 and GH 4–10 °dH. Habitat readings are snapshots, not a reason to adjust pH rapidly. A stable temperature within the specialist aquarium recommendation matters more than chasing a daily number. Hardness in FishBase is a field value; Seriously Fish does not state a specific aquarium GH. Test your source water and acclimate fish gradually. Avoid temperature swings, accumulated waste and abrupt chemical changes. Do not add salt, peat extract or pH products without a clear purpose and monitoring plan.

Behaviour and group size

Seriously Fish recommends a species tank and reports that males fight one another. A usual arrangement is one male with two or three females and adequate cover. A male may harass a single female intensely; extra females can spread attention but do not guarantee a peaceful group. Check daily that all fish feed calmly and fins remain intact. Tiny fish and fry may be eaten. This is not a shoaling fish that must be kept in a large mixed school. Given its territorial behaviour and short life cycle, no general tank-mate combinations are promoted here as suitable. Do not mix undocumented lines; N. melanospilus is a separate species, not a colour form of this fish.

Feeding

Nothobranchius guentheri is a micropredator. Seriously Fish recommends small live or frozen prey such as Daphnia, Artemia and bloodworms. Many individuals accept dried food only occasionally. Do not rely on dry food as the sole staple; use it as a supplement only when you have observed the fish eating it. Offer small portions that are quickly consumed and rotate suitable prey. Remove leftovers to protect water quality. Newly hatched fry are tiny and need suitably fine live first foods; Artemia nauplii can follow later. Check that food reaches every fish, especially when a dominant male is present.

Breeding

This is an annual killifish. Eggs are laid into peat or another breeder-used spawning medium and can tolerate a dry incubation phase. Seriously Fish describes breeding with one male and two females, daily spawning for up to several weeks, and rest periods to reduce exhaustion in females. Remove the moist medium, let it dry until damp rather than wet, and label it with species and date. Specialist sources give differing incubation durations: the BKA reports about two to three months at 24°C, while Seriously Fish gives an 8–14-week storage interval. Timing depends on temperature and moisture; monitor eggs and do not wet them prematurely. Fry need infusoria as initial food. Breed only when grow-out space, food and a responsible plan for offspring are ready.

Sources & review

This care guide draws on several specialist sources and was last checked on 17 October 2026.

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