Freshwater Fish

Black Ghost Knife Fish: A Definitive Guide For Aquarists

The black ghost knife fish has gotten a lot of attention in the fishkeeping community.  It is interesting as it is unique, and it has caught my eyes. It is one of the freshwater species that amazes me for its astounding features I don’t see every day in other fishes.

This fish has a unique appearance, so there’s no doubt that it has captured the interest of a lot of fishkeeping enthusiasts, too. When it’s well taken care of, it can grow and reach up to one foot and stay healthy.

In this guide, we’ll discuss everything to know about keeping this fish. Information will include their diet, tank size, and more!

Overview Of The Black Ghost Knife Fish

If well maintained and taken care of, you can expect your ghost to live well over a decade.  

We’ll talk more about this later. For now, let’s get to the overview of what you need to know about the ghost.

The black ghost (Apteronotus albifrons) isn’t as straightforward to care for when compared to other species due to its unique needs aside from its large size. 

They’re nocturnal fishes native to South America. The fish has poor eyesight, but they can use electrical signals to move around and feel their surroundings. 

Their electric receptors won’t stun you, though. Instead, they help the BGK in finding their food.

They are commonly found in rivers of Paraguay and Parana, so they are in countries like Brazil, Venezuela, Argentina, and Paraguay.

Their natural habitat is full of vegetation and is characterized with a lot of crannies and nooks for hiding. The water has moderate currents and murky with low lighting.

The water in their natural habitat can also be warm with neutral pH. 

On the other hand, the substrate in their natural habitat is soft sand; thus, it won’t scratch or harm the skin of the BGK.  This is very important because the fish is scaleless.

They are not the only dwellers in the rivers mentioned earlier but also a lot of insects, which are food to our featured fish. They also feed on insect larvae.

To successfully keep a knife fish black ghost in captivity, the tank should resemble their natural habitat.  

Their common name is knifefish, primarily due to the absence of a caudal or a dorsal fin, making it appear like a blade. 

The “black knife” in its name is due to their resemblance to a blade, while “ghost” comes from the belief that ghosts of dead people are in their body.

Instead of a dorsal or caudal fin, it has an anal fin. It is distinctively running along its body’s bottom edges and then rippling back and forth.

For this distinct characteristic, the BGK can easily move around even in tight spaces.

Unlike cute bettas and guppies, the black ghost knife (BGK) is a relatively large pet, so a lot of swimming and living space must be considered.

Considered a bony fish, they belong to the knifefish group and a member of the Apteronotidae family.

The fish is unique for the electrical organ in their caudal peduncles that they use to find food. This fish requires at least a 150-gallon tank along with a great filtration system.

They’re also reclusive and timid and require a lot of hiding places. That is why you must consider adding more rocks and roots in the tank.

Use fine gravel for fine substrate and subdued to dark lighting for its lights.

Some people may also prefer buying a ghost tube for their ghost fish. This is a plastic tube that aids in viewing at daytime.

Once the fish has become familiar with the tank and their surroundings, they can be trusting and tame. Some hobbyists even claim holding them in their hands.

The ghost can live with a community fish provided you combine them with peaceful and larger species. However, they tend to be aggressive towards fishes of the same size as them. They aren’t as friendly to fish of smaller sizes either.

For their diet, the BGK can feed on all types of live food, such as chopped earthworms, meat, and frozen/flaked foods.

Over the years, the fish has been growing in popularity because their appearance stands out.  A lot of them are now captive-bred and available at affordable prices ranging from $15 to $20 depending on their size.

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In aquarium specialty stores, you might find them in overcrowded tanks. For this reason, check what you’re buying and ensure you’re getting a healthy ghost black knife fish.  Look for the white spots on the tail of the fish.


A well taken care of black knife ghost fish can live for about 10 years or sometimes even more when kept in optimal conditions.  Some even claim that they can reach up to 15 years.

A few things to remember if you want your fish to have a long lifespan is to take care of them properly, including in regards to their food and diet, tank size, and tank setup.

Appearance And Size

There’s nothing that looks quite like this fish, so you’ll be fascinated seeing it in person. 

Intriguingly, they look like a knife, hence the name. They have a long and thin body that has a slight curve in it. 

The fish doesn’t have a caudal or dorsal fin. There is simply a thin ridge on top of their body and on their tail is the absence of a caudal fin. They have a skinny tail that has some white bands.

The fish can generate momentum from their anal fins and pectoral fins due to the lack of caudal fin, allowing them to move gracefully.

You can compare the movement of their anal to the stingray’s wings.  They can create a wave-like effect, giving them that impressive mobility even with poor eyesight.

For their color, the fish looks almost black. They have a ridge starting at their head running through their back, which is white in color sometimes, and they have white rings or bands on their tail.

For their size, they can grow up to 20 inches, making them not suitable for nano tanks. They need at least 120 gallons of water to thrive and survive in the tank.  They have an elongated body, which can make it hard to swim in tight spaces.

Food And Diet

The black ghost knife fish can be picky eaters and can have a hard time transitioning to pellet or flake food.

The recommended diet for this big fish should somehow be like what they’re eating in the wild.

Feed them with a natural and protein-rich diet to extend their lifespan and make them happy.

A variety of foods is recommended for your black ghost.

From what I’ve noticed, the fish love a combination of foods, such as frozen and live foods. Prawns, bloodworms, Tubifex, and brine shrimp, are great choices, too.

They don’t have any inclination on flake food, but they do eat carnivore pellets from time to time. 

Consider adding feeder fish in their diet, only when your pet is large enough. 

This fish is nocturnal, so they look for food at night. As time goes by, the fish will feel at ease to look for food during the daytime.

Consider adding a lighting system that allows subdued and moonlight settings, encouraging the black ghost to come out during times when you need to feed them.

Interestingly, they can also feed from their owner’s hand. Thus, you should keep your hands free from soap and dirt before handling the black ghost.

Feed them once a day, preferably in the evening if this works on your schedule.  Alternatively, you can also offer them food several times daily.

Avoid overfeeding your fish because it can make them fat, and it’s not good. Being fat will make them more susceptible to disease aside from putting more waste into the tank.

Know that this fish is sensitive to poor water quality, too.

Reduce the quantity of the food you’re giving them if they can’t eat all of them in a couple of minutes. 

Tank Setup And Size

The black ghost requires at least 120 gallons of water because they can grow large; thus, they need a tank that can provide them with enough space. 

Otherwise, they might show aggression if kept in a smaller tank. It can also affect their health and cause them stress. 

The tank size is as important as water quality when it comes to taking care of the ghost.  If you’re looking to add another black ghost in the tank, you should also increase the water. 

For every extra fish, you should add between 80 and 100 gallons of water. This will help reduce their aggression towards one another.

These fish are sensitive to water changes and conditions like other scaleless fish. You should invest in a high-quality filtration system with a strong flow.

To keep the water clean, you must make partial water changes of between 30 and 50 percent.  It will depend on your tank’s bioload.

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Water testing is also important to monitor the changes and fluctuations in both nitrate and ammonia levels.  And again, it will help to install a UV sterilizer, which can aid in killing any potential bacteria that might cause sickness to your fish.

Most of the time, they spend their time near the tank’s bottom, so using fine gravel and sand mix is also recommended for a substrate. This can mimic the natural environment where they live in the wild.

You must also set up your tank with thick planting around its perimeters. Consider smooth rocks and driftwood for decoration. They don’t generally eat plants, but they love hiding during the daytime.

Water Parameters

The fish is rather flexible when it comes to water parameters provided that it is clean. The water temperature should be kept between 73 degrees to 80 degrees Fahrenheit and with water pH level between 6.5 and 8. For the water hardness, it must be between 0 to 10 KH.

But even if they’re quite flexible, you should know that they can be sensitive if kept in a tank with suboptimal water conditions. They can be quite delicate to tolerate average water quality unlike other species.

Without even saying, you must pay close attention to the water quality and take it seriously if you want to keep the black ghost, which can easily catch disease. Do not keep them in a tank with poor water quality.

Aside from the clean water, you must consider the water parameters, which you must keep at optimal levels and stable all the time.  If you notice a shift, you should start fixing it.

You might want to buy a water test kit, which will help you check and monitor water quality and parameters.  Read reviews to look for the right kit that can return accurate results. This will help you decide better when treating the tank’s water condition.

Behavior And Temperament

The fish is less sociable and prefers to do things on their own. As much as possible, they want to be alone and just want to swim in and out of their hiding places to look for food when the sun sets off in the wild.

But then, they can also be aggressive especially when facing or around another fish of its own.  You don’t have to worry, though, because you can still keep at least two fishes of the same kind in a single tank.

Generally, they will be just fine provided you give them enough space even if you keep at least two black ghosts in the tank.  This will reduce their aggression and territorial behavior.

They’ll just be stressed when kept in a small space. This can also lead to their loneliness.

Compatibility And Tank Mates

You’ll also have plenty of choices when it comes to tank mates you can combine with them. The knifefish doesn’t use its size to bully or harass other fishes.

You can consider adding some peaceful fishes provided that they’re quite large.

Some hobbyists have tried pairing them with small fishes like neon tetras and celestial pearl danios. However, this can only be possible if the knifefish is still in its juvenile stage.

A few choices on compatible tank mates with the black ghostinclude the silver dollar fish,  rope fish, Oscar fish (provided they’re given enough space), cory catfish, electric blue acara,  and bichir.

There are more compatible tank mates to choose from, but these are pretty nice additions to a tank with the ghost black knife fish.

If you want to keep them with similar species, I recommend not going beyond two per tank. Otherwise, aggression is more likely even if you house them in a bigger tank. 

The reason for this is that they have poor eyesight and crowding more than two might make them bump into each other when finding food.


It can be quite difficult to breed this fish, and it is not recommended especially for beginners.

On the web, you might find a lot of misinformation regarding this matter, and this includes information of different breeding methods. It can be hard to figure out what will work and what will not.

There are claims that some fisheries in Indonesia were able to breed the black ghost; however, not much has been disclosed about the methods they used. Thus, breeding of the BGK is still a mystery to a lot of aquarists.

A few factors that may encourage breeding are a densely planted aquarium, at least 100 gallons of water, more hiding places, flooding and draining the tank, and eggs being laid in a sheltered area like a cave.

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Nevertheless, the black ghost knifefish is not a common fish for breeding in tanks even if there were hobbyists claiming to have successfully done it.

Until more reliable information on breeding the black ghost becomes available, I don’t recommend breeding them. 

Common Diseases

The fish tends to catch a skin disease when living in a tank with poor water parameters and quality.  One of the reasons for this is that they don’t have scales to protect them from the pollutants in the surroundings compared to other fishes.

Ich is a pretty common disease to watch out for.  Some signs include white spots, which you can easily recognize due to their black skin color.  They might also exhibit a change in behavior.

The fish can also contract infection if their skin gets scratched or cut.

Do you notice cuts in the skin of your fish? Notice behavioral or physical symptoms and take them seriously to reduce the chances of your fish from getting serious infection.

You must ensure that they’re healing correctly and look for treatment options if not. 

To prevent a skin infection, inspect the fish for a few minutes daily. 

The disease can be treated with an over-the-counter medication.  Remember not to use medication with copper. You can also treat it effectively through raising the temperature in the aquarium to 82 degrees Fahrenheit for four days.

Treat the whole tank once ich is active in the tank. You don’t need to isolate your fish, but to treat the water to kill the parasite causing it.  When you have controlled it, perform a water change and then reduce the temperature of the water to its usual level.

Knifefish can also be prone to worm infestations and protozoa and skin flukes. They can also be susceptible to other bacterial infections due to injuries.

To be safe, quarantine a new fish before adding it to a tank. This applies for all types of fish you wish to keep, especially if you already have a tank previously setup, allowing you to monitor any potential issues before introducing the fish to your tank. 

It may also help to treat the water in your tank using an antibacterial product to kill any disease that a new fish may have.

Frequently Asked Questions

How Long Do Black Ghost Knifefish Take To Grow?

The black ghost knifefish can take about two years to grow to their maximum size. Initially, their small size can be about two inches or five centimeters.

However, they can grow large for up to 60 centimeters or 24 inches and live at least 20 years when kept under optimal conditions.

How Can You Tell If A Black Ghost Fish Is Male or Female?

There isn’t much known about this aspect, but according to sources, males have eyes that are positioned more towards the top of their heads. The females’ eyes can be closer to the front of their head. Both males and females are also said to have glowy red tails during the courting stage.

Will A Black Ghost Knife Eat Snails?

Yes, a black ghost may eat snails. In the wild, the ghost usually hunts for food after sunset.  They are carnivores and can feed on insect larvae, insects, snails, worms, and invertebrates.

Can Ghost Knife Fish Live With Cichlids?

Even if the ghost knife fish is a semi-aggressive type of fish species, they can live well with other fishes in a community. Generally, they’re perfect to combine with catfish, cichlids, and other peaceful similar sized fishes.

Does Ghost Knife Need Air Pump?

Yes, they need an air pump because they consume much oxygen from the water. Thus, it is important to aerate and oxygenate your tank’s water.

Final Thoughts

Is the black ghost knife fish for you? After reading this guide, it’s time to reflect and ask some questions. Can you set up a large tank?  Can you keep up with the water parameters and water quality demands of this fish?

You should figure things out before deciding to buy a BGK, which is not for beginners undoubtedly. But then, if you’re an aquarist who has been in the hobby for quite some time and you think you’re ready for the black ghost, there is no stopping you.

Start investing in the right items to include in your tank, including a water filtration system, lighting system, and UV sterilizer, to name some. You must also invest in a high-quality tank of at least 100 gallons in water capacity.

Nevertheless, don’t rush and study your options carefully before deciding to keep a knifefish. Learn more about the ghost and figure out if this is the right pet fish for you. Happy fishkeeping!


Passionate fishkeeper. Nature lover. Creative thinker. Music junkie. Adventurer.

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