Black Tetra

About the species

The black tetra is one of the few aquarium fish that loses colour with age rather than gaining it: juveniles are intensely black with contrasting white fin tips, while adult fish gradually fade to a greyish colour over time. It's one of the hardiest and most adaptable shoaling fish in the hobby, suited even to a cooler, unheated tank, but it also has a less pleasant trait — a tendency to nip the fins of slower tankmates, which makes choosing tankmates less straightforward than its general hardiness might suggest.

Why it turns grey instead of staying black

Young black tetras are strikingly black, with dark grey to black fins and bright white tips on the dorsal and anal fins, which is also where the English name black widow tetra comes from. As the fish ages, the intensity of the black fades and the adult becomes noticeably greyer — this is a normal part of the species' ageing, not a sign of illness, though it often surprises owners comparing their two-year-old fish with the shop photo of a juvenile.

A subtropical species, not an equatorial one

The black tetra comes from the Paraguay and Guaporé river basins in Brazil, Bolivia, and Argentina — a region with a subtropical rather than tropical climate. As a result it tolerates a wider and somewhat cooler temperature range than many other aquarium tetras from the equatorial Amazon, making it suitable even for an unheated tank in a warm enough room.

A wide range of water parameters

The black tetra tolerates temperatures of roughly 20 to 26°C, pH of 6.0 to 7.5, and general hardness of 5 to 19°dH — one of the widest ranges among the four species covered here. That adaptability means it also does well in a tank running on ordinary tap water, without special softening or acidifying.

Fin-nipping – a trait that's easy to overlook

Despite its generally peaceful reputation, the black tetra is known for occasionally nipping the fins of slower or long-finned tankmates, such as bettas or angelfish. The risk drops noticeably in a larger shoal — a group of at least 10 to 12 is recommended rather than the 6 often suggested, since in a smaller group aggression tends to focus on individual tankmates rather than spreading out within the shoal.

Tankmates to avoid

Avoid long-finned or slow-moving species such as bettas, fancy-finned guppies, or angelfish, which are frequent targets for nipping. It pairs better with lively danios, other tetras of similar size, and calm bottom-dwellers such as corydoras catfish, which swim in a different part of the tank and rarely come into contact with a black tetra's fins.

Easy breeding

Unlike the ram cichlid, which cares for its brood as a pair, the black tetra is a typical egg-scatterer — the female simply scatters her eggs among the plants and takes no further interest in them, and the parents will often eat them if they aren't removed. Because of this, breeding in a separate spawning tank with a dense mesh or plants covering the bottom is relatively straightforward, and it's often a beginner's first experience breeding aquarium fish.

Diet

The black tetra is omnivorous and accepts almost any food in the aquarium — quality flakes and pellets, supplemented with frozen or live foods such as daphnia and brine shrimp. Overfeeding isn't a common issue, but a regular, varied diet helps the colour of young fish before it fades with age.

Longevity

The black tetra is relatively long-lived for a small tetra — with good care it reaches 3 to 5 years, and some sources report up to 10 years under exceptionally stable conditions. Combined with its broad tolerance of water parameters, that explains why the species is such a common choice for tanks that aren't tightly monitored, such as those in schools or offices.

Common mistakes

The most common mistake is keeping too many long-finned tankmates in the same tank, which raises the risk of fin-nipping. A second common mistake is assuming the fish has become boring once it turns grey as an adult — in reality that's simply natural ageing, not poor health or incorrect care.

Pros & cons