Black Ghost Knife Fish 101: Care, Tank Mates, And More

The black ghost knife fish is one of the most interesting and unique freshwater species that we’ve ever seen. When you see one in person you won’t forget it!

Because of their appearance, these fish have gotten a fair amount of attention from the aquarist community. But in our experience, there’s a lot of misinformation being passed around about this species. 

Black ghost knife fish care is not as straightforward as a lot of other freshwater species. This is partly due to their size, but also unique conditions that these fish need.

This guide will teach you everything you need to know about owning these fish. By the time you’re done reading, you’ll know their recommended tank mates, food, diet, tank size, and much more!

General Species Summary

Black ghost knife fish (Apteronotus albifrons) are a tropical freshwater fish that have been steadily growing in popularity over the years. This is mostly due to their incredibly unique appearance that makes them stand out in any tank.

They get their name from two sources. The “black knife” portion comes from the obvious resemblance these fish have to a blade (especially if you think of the white rings on their tail as a handle). The “ghost” piece of their name originates from the local belief that ghosts of the dead occupy the bodies of these fish.

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Black ghost knife fish come from a few areas in South America. The Paraná and Paraguay River are two bodies of water with a large concentration of these fish. If you follow these bodies of water they can be found in countries like Paraguay, Argentina, Venezuela, and Brazil (among others).

One interesting fact about this species that many people don’t know is that they’re electric. They’re not capable of stunning you or anything like that, but they use electric receptors to help them locate hard to find food.

Since they’re nocturnal and do most of their hunting at night, this is obviously quite a useful skill to have!


The average lifespan of a black ghost knife fish is around 10 years under solid care conditions. However, this can reach up to 15 years in some cases!

The main factors that influence their lifespan are the quality of overall care (both before you purchased your fish and after) as well as genetics. Do the best you can with the factors you can control, and you’ll likely have this fish for quite a while.


This is where things get fun.

The appearance of the black ghost knife fish is obviously the main reason why these fish are so popular. There’s really nothing like it!

A side view of the black ghost knife fish

Their bodies are long, thin, and slightly curved (like a knife). The vertical width of this fish tapers off gradually before thinning out significantly at their tail.

They don’t have a dorsal or caudal fin. On the top of their body, there’s simply a thin ridge. Their tail (where you would expect to see a caudal fin on other fish) is very skinny with a couple of spaced out white bands.

Since they don’t have a caudal fin, the black ghost knife fish generates momentum from their pectoral and anal fins. This gives them a beautiful, flowing swimming style.

One way to describe how they use their anal fin to move would be by comparing it to the wings of a stingray. There’s a wave-like effect that gives them an impressive amount of mobility.

Their pectoral fins are average-sized with a wide surface area. Because of their unique body tyle, they rely on their pectoral fins quite a bit to get around.

When it comes to coloration, there’s really not much to talk about. Other than the two white bands/rings on their tail, these fish are almost completely black. The ridge that starts at their head and runs across their entire back is sometimes a bit lighter in color, but that can vary.


The average black ghost knife fish size is somewhere between 18 and 20 inches when fully grown. This is one of the most commonly misunderstood pieces of information about this species. A lot of potential owners seem to think they’re a lot smaller than they really are!

Author Note: Don’t fall victim to the “they will grow to fit the size of their tank” school of thought that’s floating around in forums online. These fish will grow to be quite large no matter what tank size they’re in.

Black Ghost Knife Fish Care

Due to their size, unique body type, and disease sensitivity, this species is not a great fit for beginners. Black ghost knife fish care is something that’s a better fit for an aquarist who’s been around the block a few times.

This section will break down the key aspects of care when it comes to tank and water needs. Read it carefully to decide if owning these fish is right for you.

Tank Size

The minimum tank size for one black ghost knife fish is 100 gallons. These fish can grow to be quite large and need a tank that’s big enough to accommodate them.

Keeping a fish of this size in a smaller tank will not only increase their aggression, but it will cause their health to suffer as well. Having the right tank size serves as the foundation that all other care factors are built on.

You could have the best water quality in the world, but if the tank is too small it ultimately won’t matter.

If you want to keep multiple black ghost knife fish in the same tank you’ll need to increase the tank size. Aim for an additional 80-100 gallons per extra fish you add. This will minimize the chances of them showing aggressive territorial behavior toward one another.

Water Parameters

The baseline water parameter ranges that these fish can handle are actually rather flexible. This is because their natural habitat is anything but clean (hence their need to use electricity when finding food).

  • Water temperature: 73°F to 80°F
  • pH levels: 6.5 to 8
  • Water hardness: 0 to 10 KH

But even though the parameter windows are fairly flexible, these fish are actually quite sensitive to suboptimal water conditions. They are not like other hardy species that can tolerate average water quality (even though you should never settle for it).

This means you need to take water quality very seriously if you own a black ghost knife fish. They can easily get sick (more on that later) and as you probably know, the chance for disease in a tank with poor water is always high. 

The same thing goes for shifts in water parameters as well. It’s important to keep levels as stable as possible with this species. If you see a shift starting to occur it’s on you to nip it in the bud.

Author Note: Since water parameters and general quality are so important when it comes to black knife ghost fish care, you should invest in a solid water test kit. An inaccurate kit is just as bad (or worse) than no kit at all.

Make sure the levels and readings you’re getting back are accurate so you can make informed decisions about how to treat the water in your aquarium.

What To Include In Their Tank

When it comes to setting up the perfect tank for these fish, you’ll want to use their natural habitat as a reference.

In the rivers they come from, black ghost knife fish are regularly navigating various objects to hide or look for food. Trees, logs, rocks, and plants are in abundance.

This means you should try to bring some of that into their tank. This is where having the adequate tank size (or something a little larger than the minimum) comes in handy. You’ll be able to furnish the aquarium with plenty of features without it impacting their room to swim.

Before you add anything it’s important to use a soft substrate. These are bottom-feeder fish which means they’ll be spending a lot of time in the lower portion of your tank.

Fine sand or gravel is best, although we have some people get away with slightly larger gravel too (not our preference though).

The reason why a soft substrate is important for these fish is their body type. Black ghost knife fish have fairly fragile skin and no scales. This means a coarse substrate could cut them and potentially lead to an infection or illness.

Once you’ve settled on your substrate it’s time to add some hiding places.

Black ghost knife fish aren’t picky when it comes to hiding places. Any of the typical aquarium plants, rocks, driftwood, or caves will be fine.

We recommend a mixture of all of these if you have enough space. This is where you can start to get creative with how you want to set things up based on personal preference. Plants are the most important, so always try to include a few no matter what layout you decide on.

Author Note: It’s important to avoid any hiding places or objects that have rough surfaces. This means smooth rocks, caves, and driftwood. You don’t want these fish to get cut!

Diseases To Watch Out For

Because of their makeup, black ghost knife fish are more likely to get skin diseases than other species. This is because they don’t have the scaley armor that other fish can rely on.

Just like it is with most freshwater fish, Ich is always a disease to watch out for. This will show up as white spots on their skin (which will be easy to spot due to their black color) and you’ll likely notice a change in their behavior as well.

If they get cut or scratched this can also increase the possibility that they’ll get an infection. It’s important to monitor any cuts you see to make sure they’re healing properly. If the cut doesn’t appear to be making any progress you’ll need to look into treatment options.

A close up inspection of Apteronotus albifrons skin

This is why it’s so important to spend a few minutes each day inspecting your black ghost. Too often owners get into the habit of treating their aquarium like background imagery and don’t take a closer look.

Any physical or behavioral symptoms that look out of the ordinary should be taken seriously with this species. Time is of the essence.

Author Note: Investing in a great filter like the Fluval FX4 and a UV sterilizer are effective ways to reduce the chance of this fish getting sick.

The ideal diet for black ghost knife fish is very similar to what they eat in their natural habitat. Unlike some other species, these fish tend to be very resistant to transitioning over to flake or pellet foods.

Because of this, we prefer to give them what they want. From what we’ve observed, the black ghost knife fish that have lived the longest have been fed with a more protein-rich and natural diet.

Any of the go-to live or frozen foods will work (a variety is always recommended). Bloodworms, prawns, brine shrimp, and tubifex are used by owners regularly.

Since these fish are nocturnal you’ll need to work around their sleep cycle. The easiest way to do this is by feeding them once a day, and doing it in the evening or night (depending on your schedule).

Early on in your ownership, it’s crucial to make sure you’re not overfeeding them. If they can’t eat all the food you gave them in a couple of minutes, reduce the quantity.

Overfeeding will not only impact their health directly (fat fish are sick fish), but it will result in extra waste in their water. If you read our water quality section you’ll know how dangerous that can be to this species.

Behavior & Temperament

In general, black ghost knife fish want to mind their own business. They’re relatively active fish that prefer to do their own thing and be left alone.

They’re used to swimming in and out of hiding places near the substrate looking for food. Anything else is extra stress they don’t want to deal with!

However, they have an aggressive side.

This will pretty much only present itself when they’re around other black ghost knife fish. But don’t worry, it doesn’t mean you can’t keep two or more in the same tank.

As long as these fish have enough space to call their own, it’s unlikely that they’ll be aggressive toward one another. It’s just when two or more are crammed into close quarters that they can get pretty grumpy (which we understand).

Black Ghost Knife Fish Tank Mates & Compatability

There are a number of compatible black ghost knife fish tank mates you can consider. This is because this species likes to mind their own business and doesn’t use its size to be a bully.

Almost any peaceful freshwater fish can be paired with your black ghost as long as they’re not too small. Some owners have gotten away with small fish like Celestial Pearl Danios or Green Neon Tetras but that pairing tends to only be viable while the knife fish is still on the small side.

Here are some black ghost knife fish tank mates we recommend:

There are tons of other tank mates you can try, but those are a good starting point.

Author Note: If you want to keep multiple black ghost knife fish in the same tank you need to make sure they have enough space. Also, we recommend capping the number at two per tank.

Any more than this will significantly increase the potential for aggression no matter how big your tank is. This is because they have bad eyesight and will probably bump into each other when looking for food!

Breeding Information

Breeding these fish is something we don’t recommend due to the lack of information that’s available on the process. There are so many conflicting methods that have been tried that it’s hard to tell what works and what doesn’t.

Unlike some of the other guides on the internet that are fine passing along general best practices and hoping for the best, we believe it’s be a disservice to these fish to encourage owners to breed them without a reliable course of action.

In the future, if more breeding information becomes available we’ll gladly add it to this guide. Until then we don’t encourage anyone to try it in a home aquarium.

What’s Next

Black ghost knife fish care is a very rewarding process if you’re up to the challenge. It never gets old watching these fish swim around the aquarium!

Now that you have a better understanding of what kind of effort is required if you want to own this species, it’s time for you to decide if they’re right for you.

We always encourage potential owners to be as honest with themselves as possible when doing this. The worst thing you can do is purchase a fish you’re not prepared to care for (it’s not good for anyone).

If you’re on the fence and have any questions about black ghost knife fish let us know. It’s our mission to encourage and help as many informed aquarists as possible!

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