One-lined Pencilfish Care Guide: Diet, Tank Mates, Diseases, Breeding & More

Updated: December 17, 2022

The One-lined Pencilfish (Nannostomus unifasciatus) is a beautiful and peaceful freshwater fish that’s perfect for beginner aquarists.

This species is easy to care for, and can be kept in a community tank with other peaceful fish. They’re also very active, and love to swim in schools.

In this guide, we’ll teach you everything you need to know about One-lined Pencilfish care. You’ll learn about their diet, tank mates, size, and more!

Species overview

The One-lined Pencilfish (Nannostomus unifasciatus) is a freshwater fish that is found in the rivers of South America, specifically in Peru, Bolivia, and Brazil.

They inhabit fast-flowing waters with a lot of vegetation. This is something to keep in mind when setting up their aquarium as they will need plenty of plants to hide in.

The diet of the one-lined pencilfish consists mostly of small insects and other invertebrates. In the wild, they will also eat some plant matter.

One-lined pencilfish are very peaceful fish and can be kept with a wide variety of tank mates. They are a popular choice for many nano aquariums due to their small size and attractive coloration.


One-lined Pencilfish

As you can probably tell from their name, One-lined Pencilfish have a very distinctive appearance. These fish are very thin (almost to the point of looking like a pencil) and have a single line that runs the length of their body.

This line is generally a dark color (usually black) and is accented by the rest of their fins and tail which are all translucent.

The fins on these fish are all very small and include a dorsal fin, anal fin, and pectoral fins. Their caudal fin is forked and also quite small.

One-lined Pencilfish have large eyes that sit on the top half of their head. Behind these eyes, you’ll find a very small mouth.


One-lined pencilfish are relatively short-lived fish, with a lifespan of only 2 to 3 years in captivity. Of course, this lifespan can be increased with proper care.

Water quality is especially important for these fish since they are very sensitive to changes in their environment.

A stable tank with consistent parameters will go a long way in keeping your one-lined pencilfish healthy and happy, which in turn will help them to reach their maximum lifespan.


One-lined pencilfish only grow to be about 2 inches in length.


Tank Size

The minimum recommended tank size for one pencilfish is 10 gallons. If you’re looking for a freshwater fish that can fit in an average-sized tank, this is the fish for you.

If you want to keep two pencilfish in the same tank you’ll want to add at least another 10 gallons to that minimum number if you want them to thrive.

Another reason why you need to provide enough space is for the sake of enrichment and comfort. These fish like to roam and will often run gentle but steady laps around your tank. Giving them a little bit of extra space can go a long way in making sure they can comfortably turn around in the tank.

Water Parameters

The one-lined pencilfish is a freshwater tropical fish that is found in the Orinoco and Amazon River basins of South America.

They have a very specific set of water parameters that they need in order to stay healthy.

If you can provide these parameters, you will have a happy and healthy fish.

  • Water Temperature: 72-82 degrees Fahrenheit
  • pH Levels: 6.0-7.5
  • Water Hardness: 2-10 dGH
  • Alkalinity Levels: 4-8 dKH

What To Put In Their Tank

When it comes to setting up the inside of their tank, there are a few key things you need to take into account.

Pencilfish are a schooling species, which means they need to be kept in groups. This means you need to have a tank that’s at least 10 gallons in size (20 gallons is even better).

Since these fish come from South America, they prefer a habitat that has some plants. This doesn’t mean you need to go overboard, but a few pieces of driftwood and some floating plants will do the trick.

The substrate you use is also important. Pencilfish like to dig and forage around the bottom of the tank, so something soft and sandy is ideal.

As for decorations, avoid anything that’s too big or has the potential to fall over. These fish are small and delicate, so they can easily be injured by something like that.

Caves and hiding places are always a good idea since these fish like to have somewhere to retreat to when they feel threatened.

Common Diseases

The One-lined Pencilfish is a freshwater fish that is relatively hardy and resistant to disease. However, this doesn’t mean that they can’t get sick.

There are a few diseases that these fish are susceptible to, the most common being ich. This is a parasite that can affect any fish, and it will show itself as white spots on the body and fins.

Other potential diseases include infections, fin rot, and other parasites. As with any fish, it’s important to keep an eye out for anything that looks unusual.

The best way to prevent your fish from getting sick is to maintain clean and stable water conditions in their tank. A well-maintained tank will lead to healthier fish who are more resistant to disease.

Behavior & Temperament

The One-Lined Pencilfish is a shy and peaceful fish that does best in a tank with plenty of hiding places. These fish are easily intimidated, so it’s best to keep them with other peaceful fish. They’re also known to be good jumpers, so make sure your tank has a tight-fitting lid.

One-Lined Pencilfish are very shy fish

One-Lined Pencilfish are timid by nature and tend to stay hidden most of the time. They’re most active at dawn and dusk, but you may see them out and about during the day if they feel safe.

One-Lined Pencilfish are peaceful fish that do best in a community tank with other peaceful fish. They’re not known to be aggressive, but they may nip at the fins of other fish if they feel threatened.

Tank Mates

One-lined Pencilfish are small, peaceful fish that make great tank mates for a variety of other species.

These fish are shy by nature and prefer to stay in groups. As a result, they do best when kept with other peaceful fish.

One-lined Pencilfish are also sensitive to water conditions. They prefer slow-moving water with plenty of hiding places.

To help get you started, here are some compatible tank mates that tend to work well:

  • Guppies
  • Platies
  • Mollies
  • Swordtails
  • Neon Tetras
  • Endler’s Livebearers
  • Pygmy Corydoras
  • Harlequin Rasboras
  • Cherry Shrimp


The One-lined Pencilfish is a beautiful and unique freshwater fish. They’re not the easiest fish to breed, but it can be done with some patience and the right setup.

The first thing you need to do is set up a breeding tank. It should be at least 15 gallons and have a lot of hiding places. Driftwood, plants, and caves are all good options.

The temperature of the water should be between 77 and 82 degrees Fahrenheit. The pH should be between 6.5 and 7.0.

Once the tank is set up, you need to add a pair of fish. It’s best to add one male and one female, but you can add two males if you can’t find a female.

The male will start to chase the female around and build a bubble nest. Once the nest is built, the female will lay her eggs in it.

The male will then fertilize the eggs and guard them until they hatch. After they hatch, you need to remove the adults from the tank. They will eat the fry if given the chance.

You can then start feeding the fry baby brine shrimp or other small live foods.


The One-Lined Pencilfish is a great fish for the beginner aquarist. They’re very easy to take care of and are very peaceful, making them a great addition to a community tank.

Their unique look is also sure to turn some heads, so if you’re looking for a fish that will make a statement, the One-Lined Pencilfish is a great choice.

Overall, we think the One-Lined Pencilfish is a great choice for the beginner aquarist or anyone who wants a low-maintenance fish.