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Care Guide for Black Ghost Knifefish — 18-Inch Electrical Oddball Fish

We get a lot of requests for this care guide because knifefish are such unusual-looking curiosities with an undulating fin that allows them to swim both forward and backward. While there are many kinds of knifefish, the black ghost knife (BGK) is the most readily available in the pet trade and often attracts the attention of beginners. Unlike your average betta fish though, it is a considerably larger pet that requires a ton of room and may live for over a decade. Before you make the commitment, let’s talk about what it takes to keep this intermediate-level oddball fish happy and healthy in the long term.

What are Black Ghost Knifefish?

Apteronotus albifrons is a South American nocturnal fish that has weak eyesight and instead uses electrical signals to navigate its surroundings. The common name “knifefish” comes from the fact that it has no dorsal or caudal fin, giving it a blade-like appearance. Instead, an extended anal fin runs along the bottom edge of its body and ripples back and forth like a wave, allowing the knifefish to maneuver bidirectionally between tight spaces.

Most black ghost knifefish are captive-bred nowadays and may cost $15-20 or more, depending on the size of the individual. Many times, juveniles are sold in crowded tanks and may get into scrapes with other fish, so make sure to get a healthy one that has both of the white spots on its tail. Given that this fish can live as long as a pet dog, consider asking the store employee to feed them first so you can confirm your new knifefish is eating well before taking it home.

black ghost knifefish in front of pleco caves

Find a healthy knifefish that has a good weight, active behavior, and its entire tail.

How big does a black ghost knife get? They can get pretty large if you are feeding them enough, so don’t be surprised if your baby knifefish matures into a 14- to 18-inch (35-45 cm) beast.

How fast do black ghost knife fish grow? In our care, we have seen them shoot up to 8 inches (20 cm) in the first year, and then the growth rate slows down to about 2 inches (5 cm) a year.

Is the black ghost knifefish hardy? Many people call them “sensitive” fish that don’t live very long, but that hasn’t been our experience. The keys to our success include feeding them really well, making sure they don’t get picked on, and maintaining good water quality, as you would with any other fish. Usually, fish can handle one stress factor, but multiple stress factors will increase the likelihood of health issues.

Can a ghost knife fish shock you? No, it is considered to be a weakly electric fish, in contrast to a strongly electric fish like the famous electric eel that can stun its prey. The electric organ of a knifefish generates a very weak electrical field, and then it has different sensing organs that can pick up the slightest distortions to this field, similar to a radar system. This detection system helps the knifefish to navigate its surroundings, locate prey, and communicate with potential mates or intruders.

How to Set Up an Aquarium for a Black Ghost Knife

Let’s talk about tank size. A 55-gallon aquarium might be okay for the first year, but it is too small for the long term. A 75-gallon fish tank is the next upgrade that may last for a few more years. Ultimately, we recommend getting a 120- or 180-gallon aquarium for an adult black knife fish. They generally do fine with pH of 6.8–7.8, temperature between 75–82°F (24–28°C), and at least 5° (90 ppm) GH. Create a heavily decorated setup with low lighting and lots of nooks and crannies — such as clear PVC pipes, rock caves, pleco caves, large driftwood, and aquarium plants. As they grow bigger, upgrade the size of their caves and tunnels so they can feel completely safe and surrounded in their shelters.

black ghost knifefish in pleco cave

Plecos may quarrel with your knife fish over the best hides, so provide more caves if needed.

Is the black ghost knifefish aggressive? In the right conditions, they can live in a community aquarium with other similar-sized, peaceful fish. You don’t want any tank mates bullying or outcompeting them for food, such as aggressive cichlids or fast rainbowfish. Also, your adult knifefish will happily eat neon tetras and any other small fish that can fit in its mouth, so avoid any nano fish when it gets bigger.

What fish can live with a black ghost knife? Hobbyists have kept adult BGKs with Geophagus “eartheater” cichlids, peaceful catfish, angelfish, and other friendly tank mates larger than 6 inches (15 cm).

Can 2 black ghost knifefish live together? We don’t recommend it since problems tend to arise. Knifefish are territorial and don’t like sharing their space with other electric fish, such as baby whale fish, elephant nose fish, and other knifefish. Obviously, anything is doable if you have a large enough aquarium, but most people tend to keep ghost knifefish in undersized tanks.

What do Black Ghost Knife Fish Eat?

Thankfully, they are not too difficult to feed and eat a primarily carnivorous diet. Despite being a bottom dweller, they willingly go after both floating and sinking fish foods. Ours enjoy a variety of worms, bloodworms, brine shrimp, pellets that fit in their mouths, and freeze-dried foods. As mentioned before, they will predate on small fish and invertebrates once they’re old enough.

Black ghost knives can be a bit shy at first and want to spend all day in their favorite hiding spot until it’s time to eat. Therefore, many people have trained theirs to eat out of their hands or from the water surface by using floating foods.

black ghost knifefish and angelfish

Black ghost knives can be trained to swim up to the surface and eat from your hand.

Do black ghost knife fish eat flakes? Flake food may not be nutritionally dense enough for knifefish in the long run, especially since it is easy for them to get outcompeted for food by other fish. That is why we tend to prefer frozen, freeze-dried, and pellet foods to help them maintain a healthy weight.

This showpiece fish is such a fun and cool-looking species to own. If you have done a lot of research and are willing to get a monster tank for its home, the black ghost knifefish may be the right pet for you. While we do not ship fish, check our preferred online retailers to see what they have in stock.

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