One-lined Pencilfish
Nannostomus sp.
The One-lined Pencilfish (Nannostomus sp.) is a peaceful fish that's rewarding to keep. Give it a tank of at least 65 litres (17 US gallons), water held at 73-82°F, and a pH of 6.5-7.5. Always keep a group of at least 6 - they feel safest in numbers.
Captive lifespan reflects typical experience in a home aquarium, not maximum recorded age; genetics, sourcing, and care vary.
Natural distribution
South America
- South America
Regional colours match the stocking calculator's origin map.
One-lined Pencilfish Tank Setup
A tank of at least 65 litres (17 US gallons) works well. Keep the water at 73-82°F with a pH around 6.5-7.5, and set up both cover and open swimming space where they spend most of their time (Mid). Always keep a group of at least 6 of their own kind - kept alone or in pairs they tend to turn shy, nippy, or wash out in colour.
What Do One-lined Pencilfish Eat?
They're not fussy eaters - feed a varied diet, rotating good-quality dry, frozen, and gel foods rather than the same thing every day, and match the food size to their mouths.
One-lined Pencilfish Tank Mates
The One-lined Pencilfish is peaceful, which is the first thing to weigh up when you choose tank mates. It's a shoaling fish, so settle a group of 6 or more of its own kind before adding anything else - a stressed, lonely fish is far more likely to nip or hide. Before you buy, run your full stocking list through our stocking calculator. It weighs temperament, adult size, and water needs together, which a single species page can't do on its own.
Are One-lined Pencilfish Good for Beginners?
Care demand varies by line and setup. Match the care level to how often you actually test and change water.
One-lined Pencilfish - frequently asked questions
What size tank do One-lined Pencilfish need?
One-lined Pencilfish need a tank of at least 65 litres (17 US gallons). A bigger tank is always easier to keep stable, and gives them more room to settle in and behave naturally.
How many One-lined Pencilfish should I keep together?
Keep a group of at least 6. One-lined Pencilfish are a shoaling species and become stressed, lose their colour, or hide when kept singly or in pairs.
What water parameters do One-lined Pencilfish need?
Keep them at 73-82°F with a pH of 6.5-7.5, steady rather than chased to an exact figure. In a cycled tank, ammonia and nitrite should both read 0 ppm, with nitrate kept low through regular water changes.
What do One-lined Pencilfish eat?
One-lined Pencilfish are omnivores and aren't fussy eaters. Feed a varied mix of good-quality dry, frozen, and gel foods, and match the food size to their mouths. Feed small amounts once or twice a day, and remove anything they don't eat.
Are One-lined Pencilfish good for beginners?
It depends on the setup. Match One-lined Pencilfish to how often you actually test water and perform maintenance.
Common mistakes with One-lined Pencilfish
- Buying fewer than 6 - One-lined Pencilfishs need a proper school to feel secure; isolated individuals hide, fail to colour up, or become stressed.
Building a community? Run the full list through the stocking calculator before you add anything else.
Great tank mates
Chosen from temperament, overlapping temperature/pH bands, plausible minimum tank footprints, and size versus yours in App-aquatic data. Hobby nuance always wins - verify stocking and temperament before purchasing.
Check One-lined Pencilfish against any other fish → Compares water parameters, predation risk, fin nipping and tank size, and draws both at life size.
Fish to avoid mixing with One-lined Pencilfish
These well-known species are commonly sold alongside this fish but are not recommended tank mates - verify before purchasing.
- OscarPredator - Oscar will eat or injure smaller tank mates
- Pea PufferPredator - Pea Puffer will eat or injure smaller tank mates
- Common GoldfishGoldfish - prefer cooler, harder water that tropical fish cannot tolerate
