Tetras and Pencilfish: Little Fish That Need a Crowd

Small, Colorful and Peaceful

Tetras are some of the most recognizable aquarium fish: the neon tetra's electric blue stripe, the cardinal tetra's deep red, the black skirt's flowing fins. Most come from South America, some from Africa, and OATA notes that most in the trade are now captive bred and undemanding of water chemistry.

Pencilfish are their slender, quieter cousins, mostly from South America too. Both suit a peaceful community tank with plants, and both look their best in a big group.

A Proper Shoal

Tetras and pencilfish are shoaling fish. OATA recommends keeping at least five, and more is better: a lone tetra or a group of two or three is often stressed and hides. A shoal of ten or more of one species looks far better than a few of many species.

Most tetras stay small, about 0.8 to 2 inches (2 to 5 cm), though some larger species reach nearer 4 inches (10 cm). Pencilfish rarely grow beyond 2 inches (5 cm).

Tank and Water

OATA recommends at least about 12 gallons (45 liters) for a small shoal of a small species, and at least about 21 gallons (80 liters) for larger species of 2 inches (5 cm) or more. Bigger tanks are more stable, so go as large as you reasonably can.

• Heater: keep the water at about 72–81°F (22–27°C).
• Filter: always, with a gentle current for small species.
• Plants and cover: live plants are ideal; tetras color up when they feel secure.
• Water: pH about 6.5–8.0, soft to medium hardness, ammonia and nitrite at zero.
• Cycle first: see cycling a tank.

Feeding Tetras

Tetras are omnivores. OATA suggests a good-quality flake or small granule, with small live or frozen foods such as daphnia or bloodworm as extras. Feed only what they eat in a few minutes, once a day; overfeeding fouls the water.

Tank Mates

Small species such as neon tetras and Beckford's pencilfish shouldn't share a tank with any fish big enough to eat them, which includes adult angelfish. Some boisterous tetras, such as the Buenos Aires tetra, nip fins, so keep them away from long-finned fish like bettas and fancy guppies. Good company includes corydoras, rasboras, small peaceful gouramis and otocinclus. See community tropical tanks.

Keeping Them Healthy

Most problems in small fish start with water quality. Test ammonia, nitrite and nitrate weekly, change about a quarter of the water each week with conditioned tap water, and rinse filter media in old tank water, never under the tap. Quarantine new fish if you can, and add them a few at a time so the filter keeps up.

Watch your fish every day at feeding time. Changes in swimming, color, breathing or hiding are early signs that something is wrong, so test the water first.

For fish treatments or medicines: see your vet or a pet supply store with trained staff. An aquatics store with trained staff, or a vet who treats fish, can help you work out what's wrong before you add anything to the tank.