Pencil Fish (Nannostomus) – Species Profile & Care Guide

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Pencil fish (Nannostomus) are small, colorful, and are fit for community tanks. It’s not hard to understand why you like them, at least in theory. That’s because actually caring for them may prove more challenging than it may seem at first.

Although it is a resilient fish species, it is also sensitive when it comes to vast environmental changes and fluctuating water parameters. This is why Pencil fish rank as intermediate in terms of care level.

So, what should you know about this species before deciding whether to get it or not?

Tank Size for Pencil Fish

I would say that a shoal of 10 Pencil fish requires at least 30 gallons of water, but this number care vary greatly. You need to consider several aspects before setting up your first Pencil fish tank. These include:

  • The Pencil fish temperament – These are energetic fish who enjoy swimming over longer distances and investigating their environment. For this reason, they might require more water volume than other species, despite their smaller size.
  • Setting up a community tank – When setting up the community tank, you need to consider every species’ space requirements. Community tanks automatically require more space than species-only environments.
  • The overall setup – A well-designed aquatic environment will include more than the fish themselves. You will also need to include live plants, rocks, caves, driftwood, and other aquatic decorations meant to provide the fish with a home-like environment. All these will take up aquarium space, requiring you to consider the available space more thoroughly before getting the fish.
  • Future plans – I suggest getting a larger-than-necessary tank if you plan on breeding the Pencil fish or planning on upgrading your Pencil fish-only tank to a community one.

If you can’t afford a 30-gallon tank or don’t think you need one, a 20-gallon one should do. Just make sure you provide your Pencil fish with stable water parameters and monitor their activity regularly to make sure they are healthy and thriving.

Water Requirements for Pencil Fish

Ideal water temperature sits at around 71 to 80 °F with small variations. The pH and water hardness are typical for most aquarium fish, with values between 5.7 to 7.0 and 3 to 10 dGH, respectively.

Aside from these basic parameters, Pencil fish also need:

  • A darker substrate – Using dark sand as substrate will make the fish feel more comfortable and boost their coloring over time.
  • Low lighting – Pencil fish live in low-lighting conditions in the wild. Replicating these conditions in their tank will help them feel more comfortable throughout the day.
  • Driftwood – Pencil fish love driftwood, as well as other tank decorations. They will use these decorations to hide and rest during both daytime and nighttime.

What do Pencil Fish Eat?

In the wild, Pencil fish will typically prefer zooplankton with your occasional invertebrates on the side. Unexpectedly enough, the fish’s diet is more varied in captivity, as the Pencil fish has adapted to eating pretty much anything.

The Pencil fish is omnivorous, so feel free to feed it anything that falls in the category of ‘fish food.’ This includes fish flakes, pellets, frozen or dry food, and even live animals like bloodworm and brine shrimp.

An important note here is that Pencil fish have small mouths. You need to break their food into smaller pieces so they can consume it. Otherwise, they might injure their mouths or even choke on it.

Do Pencil Fish Need a Heater?

I would say yes, they do. The reason for that is that the Pencil fish is sensitive to environmental fluctuations, especially temperature, and the temperature can sometimes fluctuate without realizing it.

This is a problem, especially in rooms that don’t have a steady heating source to keep the temperature stable. In that case, a heating system is necessary to balance the temperature and keep it within ideal parameters. I would say that having a heater in place is a must, no matter how stable the temperature outside the tank is.

You never know when the room’s temperature can fluctuate due to a door left open during colder seasons, affecting the tank’s Fahrenheit values as well.

Do Pencil Fish Need a Filter?

Yes, they do. A filter is necessary to keep the water clean, fresh, and well-oxygenated, which is a must for any Pencil fish population. The filter is that much more necessary for community tanks with several fish species that will produce a lot of waste.

The Pencil fish requires pristine water conditions, and community tanks are notorious for displaying elevated ammonia levels due to excess food and fish waste. The filter will keep the water cleaner for longer, providing the fish with adequate aeration, water movement, and cleaning any water debris and floating residues.

A good filtering system is a nice addition to your tank cleanup and maintenance tools, lowering the need for too often water changes.

Are Pencil Fish Aggressive?

No, not at all. Pencil fish are friendly and shy and will get along with any fish species, so long as it’s as peaceful and acceptant. The Pencil fish’s timid personality is incompatible with the bullying or territorial behavior that other fish species will display.

If you’re looking to introduce the Pencil fish to a community tank, make sure you find them compatible tank mates, to reduce their stress and keep them safe. Some of the most valid candidates include tetras, barbs, guppies, mollies, platies, etc.

Avoid pairing them with aggressive, carnivorous, or large fish species that may bully or even kill and eat Pencil fish. As a general rule, the ideal tank mates should match the Pencil fish in terms of size, diet, behavior, and tank requirements.

Do Pencil Fish Make Good Tank Mates?

Pencil fish are great for community tanks, so long as they’re paired with compatible fish species. You should choose their tank mates based on size, diet, and behavior, like I’ve mentioned above. Fish species like large cichlids or male Bettas are unfit since they display overly aggressive behavior and can kill the Pencil fish.

It’s also worth noting that constant harassment will disrupt the Pencil fish’s lifestyle, forcing it into hiding and stressing it out. This can weaken its immune system and make the fish prone to various parasites and bacterial infections.

Just to be sure, you should decorate your tank with various aquatic elements like driftwood, rocks, and caves. These will provide the Pencil fish with hiding and resting places in case they feel threatened or stressed.

Are Pencil Fish Schooling Fish?

Yes, Pencil fish are a schooling species. For this reason, you should only keep them in schools of at least 10 individuals. This will provide them with comfort and peace of mind, minimizing the male aggression during the mating season.

How do You Breed Pencil Fish?

The breeding process is slightly more demanding for Pencil fish than other species. If you plan on breeding your Pencil fish, here’s what to consider:

  • Impeccable water conditions – Pencil fish tend to become even more sensitive to the water’s quality during the reproductive season. Make sure you provide them with good aeration, regular water changes, and a clean and stable environment. This will also benefit the fry.
  • Dietary changes – Breeding females should receive live food for about 7 to 10 days prior to the breeding season. The surplus of protein will provide them with the necessary nutrients to carry on with the mating process, increasing the chances of successful pregnancies.
  • Consider a breeding tank – Pencil fish can grow aggressively during the reproductive phases. I suggest preparing a breeding tank to keep the females until they lay their eggs and remove them afterward. This will eliminate the risks of the Pencil fish eating their own eggs.

As additional advice, keep the breeding tank in low light and decorate it with plenty of live plants. Each female will typically lay around 30 eggs which will hatch within the following 35 hours. The resulting fry will require a surplus of protein, most of which should come from micro-worms and brine shrimp.

Just make sure to cut their meals into smaller bits so they can eat them.

How to Treat Sick Pencil Fish?

The Pencil fish is prone to a variety of conditions, most of which are the direct result of improper tank conditions and stress. These include bacterial infections, worms, and skin flukes, to name a few.

The best approach is prevention when it comes to Pencil fish care. Provide them with optimal water conditions, pair them with compatible and friendly tank mates to minimize their stress. You should also provide them with a lush aquatic environment filled with plants, tank decorations, and hiding spots.

Treatment-wise, you should move fish suspected as sick in a quarantine tank. This will allow you to identify the problem without risking the health of the entire fish population. After that, the nature of the treatment will depend on the nature of the disease.

Speak to a professional if you’re unsure how to address their condition effectively.


Pencil fish are a rather demanding species in terms of care and environmental conditions. If you’re a beginner in the aquarium business, Pencil fish may not be the ideal choice as your first fish.

If you’ve decided you can care for them, consider this guide as a starting point.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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