Red Arc Pencilfish 101: The Ultimate Care Guide

Red Arc Pencilfish is a species of freshwater fish that is native to the Amazon Basin in South America. 

Specifically, they live in the tributaries and streams that flow through Brazil, Peru, and Colombia.

These fish have a striking physical characteristic – a bright red arc that runs along their body, resembling a pencil line. 

In this guide, I’ll cover all the essential information you need to know to provide optimal care for your Red Arc Pencilfish, including their habitat, behavior, diet, breeding habits, and much more. 

Red Arc Pencilfish Summary

Common NamesRed Arc Pencilfish, Mortenthaler’s Pencilfish
Scientific NameNannostomus mortenthaleri
SpeciesNannostomus mortenthaleri
OriginRio Negro, Brazil
HabitatSlow-moving streams and flooded forest areas
ColorBright red with a black spot on the caudal peduncle
SizeUp to 1.5 inches (3.8 cm)
LifespanUp to 5 years in captivity
pH Level5.5-7.5
Water Temperature73-82°F (23-28°C)
Water HardnessSoft to moderately hard
Water TypeFreshwater
Minimum Tank Size10 gallons (38 liters)
Community TankYes, peaceful community tank
Care LevelEasy
CompatibilityPeaceful community fish, avoid keeping with aggressive or fin-nipping species.

Red Arc Pencilfish History

The Red Arc Pencilfish is a relatively new addition to the aquarium hobby as scientists described it as a distinct species only in 1987. 

Its scientific name, Nannostomus mortenthaleri, honors the contribution of Richard Mortenthaler, an aquarist, and breeder who was instrumental in the discovery and propagation of the species in the aquarium hobby.

Today, aquarists breed these cuties in captivity and they are widely available in the aquarium trade. 

Red Arc Pencilfish Origin & Habitat

The Red Arc Pencilfish (Nannostomus mortenthaleri) is native to the Rio Negro, a blackwater river in the Amazon basin of Brazil. 

The Rio Negro has a unique water chemistry, which is characterized by a low pH range of 4.0 to 5.5 and a low mineral content. 

This type of water is often referred to as blackwater due to its tea-like color and acidic nature.

In their natural habitat, these fish dwell in slow-moving streams and flooded forest areas. 

They prefer the upper and middle levels of the water column and often swim in areas with dense vegetation or fallen logs. 

These species are also shoaling fish, forming groups of individuals that swim together in the wild. 

Red Arc Pencilfish Behavior

Red Arc Pencilfish are a shoaling species and are social in nature, which means they feel most comfortable and secure when kept in a group of at least 6-8 individuals. 

When they’re alone or in smaller groups, they may become shy and hide more often, which can lead to stress and health issues.

They can also be fin nippers if they live in inadequate conditions or with inappropriate tank mates. 

In the aquarium, these guys are active swimmers and spend most of their time in the middle and upper levels of the water column. 

But, don’t worry they’re not jumpers and can be kept in an open-top aquarium. 

They prefer a well-planted environment with plenty of hiding places and driftwood, which can help to replicate their natural habitat and make them feel more comfortable.

What Are the Features of Red Arc Pencilfish?

1. Appearance

The Red Arc Pencilfish have a slender, pencil-like body shape, with a slightly pointed head and a pointed tail fin.

Their striking coloration is one of the most distinctive features of the species. 

The bright red coloration extends from the head to the tail, with the intensity of the color varying among individuals. 

Some may have a more intense coloration, while others may be a lighter shade of red. 

The red coloration is often described as fiery or electric in appearance and makes this species stand out in any aquarium.

Another distinctive feature is the black spot located on their caudal peduncle, which is the area just before their tail fin. 

This spot is particularly prominent in male individuals and is thought to be a sexual signal to attract females during mating. 

The females also have this spot, but it is typically smaller and less prominent than in males.

The fins of these pencilfish are transparent, with a hint of red coloration on the dorsal and anal fins. 

The caudal fin, or tail fin, is forked and is also transparent. 

The males have slightly longer fins than the females, which is another way to distinguish between the sexes.

2. Body Size

Typically growing to around 1.5 inches in length, the Red Arc Pencilfish has a slender, pencil-like body shape that’s just begging for a nickname like “Slim Jim” or “Pencil Pete”. 

Now, some folks might look at the Red Arc Pencilfish and say, “Hey, that fish is too small for me. I need something big and bold!” 

But let me tell you, size isn’t everything. These little guys may be small, but they’re packed with energy and spunk. 

They’re always swimming around and exploring their environment, and they’re not afraid to stand up to larger fish.

Red Arc Pencilfish Lifespan

These charming fish can live for around 3-5 years in captivity, which may not seem like a lot, but it’s still enough time to create some great memories together.

Now, I know some of you may be thinking, “Hey, I want a fish that’ll outlive me!” But let’s be real here – these fish are small but mighty, and they pack a lot of personality into their relatively short lifespans. 

Red Arc Pencilfish Life Cycle

1. Egg Stage

During the egg stage, the female Red Arc Pencilfish lays small batches of eggs that are usually attached to the leaves of plants or other surfaces in the aquarium. 

The eggs are small and adhesive, which allows them to stick to the surface they were laid on. 

After that, the male fertilizes them by releasing sperm over the eggs.

Then the eggs will start to develop for around 2-3 days, after which the hatchlings, also known as fry, emerge. 

2. Fry Stage

The fry are very small and delicate, typically measuring around 2-3 mm in length. 

This stage is crucial for the survival and healthy development of the fish, as they are still very vulnerable at this point in their life cycle.

To care for the fry, ensure that the temperature of the water is around 26-28°C, and the pH level is between 6-7.5. 

You should also keep the water clean and well-oxygenated, as any fluctuations in water quality can be detrimental to the fry’s health and survival.

In terms of feeding, you should give these new babies small amounts of food several times a day. 

You can feed them infusoria, which are small aquatic organisms, and crushed flakes which are both suitable options for feeding fry at this stage.

3. Juvenile Stage

The juvenile stage of the Red Arc Pencilfish is an important developmental stage for the fish, as they are growing and becoming more robust. 

At this stage, the fish have grown to around 1 cm in length and are less delicate than they were during the fry stage. 

However, it’s still important to provide appropriate care to ensure their continued healthy development.

In terms of feeding, you should start to feed them live or frozen daphnia or brine shrimp. But ensure that the food is appropriate for their size, as they still have relatively small mouths.

4. Adult Stage

At this stage, the fish have grown to their full size, which is around 4 cm in length. 

They are more robust and can tolerate a wider range of water conditions than they could during the earlier stages of their life.

You should feed these guys now a more varied diet than they could during the earlier stages of their life. 

Flakes, pellets, and live or frozen foods are all suitable options for feeding adult fish. 

Is Red Arc Pencilfish Hardy?

Red Arc Pencilfish is one tough little fish. These guys are famous for being hardy and adaptable, which makes them a great choice for beginner fish keepers. 

They can tolerate a wide range of water conditions and are relatively low-maintenance compared to other fish species.

Of course, like any fish, they still require proper care and attention to thrive. But if you’re willing to put in the effort, these little guys will reward you with their vibrant colors and entertaining antics in the aquarium.

How to Care for Red Arc Pencilfish?

a.Water Requirements

1. Water Quality

In their natural habitat, these fish inhabit slow-moving, well-oxygenated streams and rivers with clear water. 

Therefore, it is important to replicate these conditions as closely as possible in the aquarium to ensure their well-being.

To do this, you can perform 20-30% water change per week to remove any excess waste, uneaten food, and other organic matter from the water. 

This helps to prevent the buildup of harmful toxins and bacteria that can be detrimental to the health of the fish.

For example, fish produce ammonia as a waste product, which can be toxic to them if it builds up in the water. 

In a well-maintained aquarium, beneficial bacteria will convert ammonia into nitrite and then nitrate, which is less harmful to fish. 

When you perform water changes, you should use a good-quality water conditioner to remove any chlorine or chloramines that may be present in tap water. 

In addition to water changes, it is also important to regularly clean the aquarium substrate and remove any uneaten food and waste from the tank. 

2. Water Temperature

In their natural habitat, these fish are typically found in warm, tropical waters with temperatures ranging from 73-82°F (23-28°C). 

Any fluctuations in water temperature can be stressful for Red Arc Pencilfish and can lead to health issues such as lethargy, loss of appetite, and even disease. 

To prevent temperature fluctuations, you can use a good-quality aquarium heater and thermometer to monitor and maintain a consistent water temperature.

When you choose an aquarium heater, you should select one that is appropriate for the size of your aquarium and the desired water temperature range. 

You should also use a backup heater in case the primary heater fails, to prevent sudden temperature drops.

In addition to maintaining a consistent water temperature, it is important to avoid rapid temperature changes, which can be harmful to the fish. 

For example, adding cold water to the aquarium can cause the temperature to drop quickly, while adding hot water can cause it to rise rapidly. 

So, you can do gradual temperature changes of no more than 2-3°F per day to prevent stressing your buddies.

3. pH Level

Red Arc Pencilfish are native to soft, acidic waters with a pH range of 5.5-7.5, and they can be sensitive to changes in pH levels.

A pH level that is too high or too low can cause stress to the fish, leading to a weakened immune system and increased susceptibility to disease. 

To maintain a stable pH level, you can use a good-quality pH test kit to regularly monitor the water conditions.

 If the pH level is too high or too low, appropriate measures should be taken to adjust it gradually. 

One way to lower pH levels is by adding natural materials such as driftwood, peat moss, or almond leaves to the aquarium. 

Another method is to use pH-lowering additives, such as pH buffers or acidic substrates.

On the other hand, if the pH level is too low, you can increase it gradually by adding a pH-raising substrate or using pH-raising additives. 

But note that making sudden and drastic changes to the pH level can harm the fish, so you should make adjustments gradually over time.

It is also important to note that different fish species have different pH preferences, so if you plan to keep Red Arc Pencilfish with other fish species, it is important to choose compatible species with similar pH requirements.

4. Water Hardness

Water hardness is a measure of the mineral content in the water, specifically the concentration of calcium and magnesium ions. 

These fish are native to soft to moderately hard water, with a water hardness range of 5-12 dGH. 

Water that is too hard or too soft can the fish and lead to health issues such as weakened immune systems, decreased appetite, and even death.

To maintain the appropriate water hardness, you can use a good-quality water test kit such as the API GH & KH Test Kit or the API Freshwater Master Test Kit.

If you find the water hardness too high, you can lower it gradually by using a water softening agent, such as reverse osmosis (RO) water or peat moss, or by mixing soft water with hard water.

On the other hand, if the water hardness is too low, you can increase it gradually by adding minerals such as calcium and magnesium to the water. 

You can achieve this by using a mineral supplement, such as API Proper pH 7.5, or by adding a substrate that contains minerals, such as crushed coral or aragonite sand.

b.Tank Requirements

1. Tank Size

Although Red Arc Pencilfish only grow up to 1.5 inches (3.8 cm) in length, they are active and social fish that require plenty of swimming space. 

Therefore, a small group needs at least a tank size of 10 gallons. 

I actually prefer maintaining a larger tank, such as a 20-gallon long or a 30-gallon tank to provide more swimming space and allow for a greater number of fish to live together without overcrowding.

A larger tank also helps to maintain stable water conditions, which is crucial for the health and well-being of the fish. 

This is because a larger water volume helps to dilute any toxins or fish waste, making it easier to maintain good water quality. 

It also provides more surface area for beneficial bacteria to grow, which helps to maintain a healthy nitrogen cycle in the aquarium.

2. Decorations

Decorations in an aquarium not only enhance its visual appeal but also provide hiding places, food sources, and breeding grounds for fish.

One popular decoration for aquariums is driftwood as it provides a natural and organic look to the aquarium and also serves as a hiding place and food source for Red Arc Pencilfish. 

But make sure to choose driftwood that has been properly cured and is free of any toxic substances or harmful bacteria. 

Boiling or soaking the driftwood for several days can also help to remove any excess tannins and make it safer for use in the aquarium.

Another popular decoration for aquariums is live plants. Live plants provide oxygen, remove excess nutrients, and create a natural environment for your fish. 

Some suitable plant species include Java moss, Anubias, and Amazon sword.

Other aquarium decorations that can be suitable for Red Arc Pencilfish include rocks, caves, and artificial decorations.

Whatever you choose, ensure to choose items that are made of safe materials and do not have any sharp edges or small parts that can be harmful to the fish.

3. Lighting

Just like us, Red Arc Pencilfish rely on a proper day-night cycle to regulate their internal rhythms and maintain good health. 

Adequate lighting helps to promote healthy behaviors and can even encourage breeding in the fish.

When selecting lighting for your tank, choose a light fixture that is appropriate for the size of the tank and the type of plants you have. 

I prefer using LED lights for aquarium lighting because they are energy-efficient, long-lasting and provide a full spectrum of light that is beneficial for both fish and plants.

You should also provide a photoperiod of 8-12 hours of light per day, mimicking the natural day-night cycle. 

This helps to regulate the fish’s circadian rhythm and can also help to prevent algae growth in the aquarium. 

In addition to providing adequate lighting, you should consider the color temperature of the lights. 

The ideal color temperature for your little buddies is 6500 K for a natural and balanced appearance.

Red Arc Pencilfish Compatibility

When it comes to compatibility, the Red Arc Pencilfish is a fairly peaceful species that can coexist with a variety of other fish. 

However, there are a few things to keep in mind when choosing tankmates for this species.

Firstly, you should avoid pairing them with any aggressive or territorial fish, as they can be easily bullied or intimidated. 

You should also avoid pairing them with fish that are too large or too small, as they may be seen as potential predators or prey.

Sometimes, these guys can display a bit of aggression towards their own kind, particularly during the breeding season. 

For this reason, you should keep them in small groups of 3-6 individuals, rather than in pairs or singly.

Here’s a table of good and bad tank mates for Red Arc Pencilfish:

Good Tank MatesBad Tank Mates
TetrasAggressive or territorial fish
RasborasLarge or small fish
GuppiesFin-nipping species
CorydorasBottom-dwelling fish that stir up substrate
LoachesFish that require vastly different water conditions
Small peaceful schooling fishRed Arc Pencilfish kept in pairs or singly during breeding season

Red Arc Pencilfish Feeding & Diet

Red Arc Pencilfish are omnivores that feed on a variety of small organisms, including insects, crustaceans, and worms. They also consume algae and other plant matter.

Their feeding habits are influenced by their environment and the availability of food sources. 

For example, during the rainy season, the increased water flow can bring in a variety of small organisms, which can provide an abundance of food for the fish. 

In times of low food availability, Red Arc Pencilfish may resort to feeding on detritus and other organic matter found on the bottom of the stream or river.

In captivity, you can feed them a variety of foods, including:

  • High-quality flakes and pellets
  • Bloodworms
  • Brine shrimp
  • Daphnia 
  • Mosquito larvae 
  • Small aquatic insects

When it comes to feeding frequency, you should feed them small amounts of food several times a day, rather than one large meal. 

This is because they have small stomachs and can’t digest large amounts of food all at once. 

Red Arc Pencilfish Breeding

Breeding Red Arc Pencilfish is relatively easy in a home aquarium if you provide them with the right conditions. 

The first step in breeding these fish is to ensure that you have a healthy and well-fed pair of adult Red Arc Pencilfish. 

I recommend keeping several males and females together in a community tank to encourage the pairing process.

To encourage breeding, provide a suitable environment for the fish, including:

  • Provide plenty of hiding places, such as caves, rocks, and driftwood, for the breeding pair to lay their eggs and for the fry to hide once they hatch
  • Maintain the water temperature at 76-82°F (24-28°C) and the pH level at 6.0-7.5 to ensure the correct environment for breeding
  • Use a good-quality water test kit to monitor the water parameters and ensure that they are maintained at the correct levels
  • Feed the breeding pairs a high-quality diet that includes a variety of foods, such as live or frozen brine shrimp, daphnia, and bloodworms
  • Simulate the rainy season in their natural habitat by gradually increasing the temperature of the water, increasing the frequency and duration of water changes, and providing a high-quality diet to encourage breeding

Once a pair has paired up and is ready to breed, they will begin to display courtship behavior. 

The male will chase the female around the tank and display his bright colors to attract it. 

Once the female is ready to lay its eggs, it will choose a suitable location in the tank, such as a plant or rock, to lay the eggs.

The eggs will be very small and transparent and can be difficult to see in the aquarium. 

Once the new mummy lay the eggs, you should remove the breeding pair from the tank to prevent them from eating the eggs or fry.

The eggs will hatch in approximately 24-36 hours, and the fry will become free-swimming in 3-4 days. 

At this point, it is important to provide the fry with a suitable food source, such as liquid fry food or infusoria, until they are large enough to eat larger food particles.

Red Arc Pencilfish Common Diseases

Like all fish, Red Arc Pencilfish is susceptible to a range of diseases. Here are some common diseases that can affect Red Arc Pencilfish:

Ich (Ichthyophthiriasis)White, raised spots on body and fins, scratching against objectsParasitic infection caused by Ichthyophthirius multifiliisMedication (e.g. malachite green, copper sulfate) and addressing the underlying cause (e.g. poor water quality, stress)Regular water changes, maintaining good water quality (e.g. appropriate temperature, pH, and ammonia/nitrite/nitrate levels), quarantine new fish before adding them to the aquarium, avoid overcrowding and overfeeding
Fin Rot (Columnaris)Deteriorating fins, frayed or disintegrating, redness at the base of finsBacterial infection caused by poor water quality, stress, or injuryMedication (e.g. antibiotics) and improving water qualityRegular water changes, maintaining good water quality (e.g. appropriate temperature, pH, and ammonia/nitrite/nitrate levels), quarantine new fish before adding them to the aquarium, avoid overcrowding and overfeeding
Velvet (Oodinum)Gold or rust-colored dusting on the body, lethargy, loss of appetite, rapid breathingParasitic infection caused by OodiniumMedication (e.g. copper sulfate, formalin) and addressing the underlying cause (e.g. poor water quality, stress)Regular water changes, maintaining good water quality (e.g. appropriate temperature, pH, and ammonia/nitrite/nitrate levels), quarantine new fish before adding them to the aquarium, avoid overcrowding and overfeeding
Dropsy (Ascites)Bloated abdomen and raised scales, lethargy, loss of appetiteSymptom of bacterial infection, poor water quality, or organ failureTreatment depends on underlying cause (e.g. antibiotics, water changes, improving water quality, supportive care)Regular water changes, maintaining good water quality (e.g. appropriate temperature, pH, and ammonia/nitrite/nitrate levels), quarantine new fish before adding them to the aquarium, and avoid overcrowding and overfeeding
Swim Bladder DisorderUncontrollable floating or difficulty swimming, swimming upside downCaused by poor water quality, overfeeding, or injuryTreatment depends on the underlying cause (e.g. fasting, peas, improving water quality, supportive care)Regular water changes, maintaining good water quality (e.g. appropriate temperature, pH, and ammonia/nitrite/nitrate levels), avoid overfeeding (feed small amounts multiple times a day), provide a varied diet, avoid rough handling and injury


How Big Do Red Pencilfish Get?

Red Pencilfish typically grows to be around 1.5 inches in length.

Is Pencilfish Hardy?

Pencilfish is a hardy and adaptable fish, able to thrive in a variety of aquarium conditions.

What Is the Really Red Pencilfish?

The Really Red Pencilfish is another name for the Red Arc Pencilfish (Nannostomus mortenthaleri), which is known for its striking and vibrant red coloration.

How Many Pencilfish Should be Kept Together?

Pencilfish should be kept in groups of at least 6-8 individuals. This helps them feel more comfortable and secure in their environment and can help to reduce stress and promote overall health and well-being.


As promised, we’ve covered all the essential information from their origins and natural habitat to their behavior, diet, and breeding habits, we’ve explored every aspect of these fascinating fish. 

Just remember, maintaining good water quality, providing a suitable environment, and feeding your fish a high-quality diet are the keys to keeping them happy and healthy. 

Do you still have questions? If so, leave a comment and I’ll be happy to answer them.

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