The oxymoron of unicornfish, neither the male nor the female Sleek Unicornfish will develop horns, however their very cool black tongue and “safer” tail spines make up for this!

The Sleek Unicornfish is a very large, yet elegant tang that can be grayish brown to greenish brown with a more pale version of those colors on the belly area.  They do not form a horn, but have a short groove in front of each eye.  Their scalpels, found in a set on each side of their caudal fin area, are fixed and very weak.  Their skin actually has a velvety look due to the granulated skin.  At 10” they form a black tongue and by 17.7” they are sexually mature with males eventually becoming just under 30” long and females closer to 20” long.  This large fish will reach 80% of their adult size by the time they are 5 years old.  They live 30 to 45 years and due to the large tank size needed are best kept by intermediate aquarists.

This Naso tang is one of the largest tang to be sold in the aquarium industry (over 2 1/2 feet in length).  In the wild, the Silver-Blotched Unicornfish (Naso hexacanthus), which is usually a solitary Naso, has been observed swimming with schools of Sleek Unicornfish.  This is done to take advantage of being able to eat zooplankton safely, after all “safety in numbers” applied to fish too!  The differences are easy to spot, as indicated by the blotches and slight nose protuberance, both of which that the Sleek Unicornfish lacks. 

For obvious reasons, the ultimate size of this fish is the challenge that aquarists face.  Although the Sleek Unicornfish is a flashy and impressive display fish, only the largest tanks can house it, which should be at least 360 gallons.  This presents the biggest issue since they should be put in this size tank even as a small 4” juvenile.  Why, well in 4 short years they will be 20” long, and most hobbyists do not want to buy a larger tank every year this fish grows 4!”

The Sleek Unicornfish is a peaceful fish toward non-tangs, getting along with most other marine fish and can be kept with a variety of tank mates including other genus’ of surgeonfish. However unless you have a huge (500 gallon plus) system, it is best to house just one of these large tangs to a tank. The Sleek Unicornfish not considered reef safe as this big rambunctious fish will topple corals while swimming about in fast clips. Though it may eat some algae it will very likely nip on small worms and possibly clam and small shrimp invertebrates as well. This pretty specimen is most suitable for an extremely large show tank or a public aquarium.

The Sleek Unicornfish requires a lot of water turbulence highly saturated with oxygen rather than a placid aquarium. Being very active during the day they need a large tank with plenty of room to swim, but will also need some rocks/ corals with crevices for retreat and to sleep in at night. In the ocean they feed on zooplankton, some benthic weeds and algae and to a lesser degree some red filamentous algae.  Aquarium fare can include such things as various worms, shrimps, and algae. Once they get acclimated and become accustom to aquarium foods they are quite hardy and good eaters.  They need a place to bunk at night like a large crevice or cave that is slightly larger, since they erect their fins and wedge themselves in at night.  

Scientific Classification

Species: hexacanthus

Sleek Unicornfish – Quick Aquarium Care

Aquarist Experience Level:Intermediate
Aquarium Hardiness:Very Hardy
Minimum Tank Size:360 gal (1,363 L)
Size of fish – inches:75.0 inches (190.50 cm)
Temperature:72.0 to 78.0° F (22.2 to 25.6&deg C)
Range ph:8.1-8.4
Diet Type: Carnivore
Sleek unicornfish
Image Credit: orlandin, Shutterstock

Habitat: Distribution / Background

The Sleek Unicornfish, Naso hexacanthus, was described by Bleeker in 1855.  The common names they are known by are Blacktongue Unicornfish, Sleek Unicornfish, Surgeonfish, Thorpe’s Unicornfish, and  Unicornfish.  These are descriptive of the characteristics of this fish.

They are found in the Indo-Pacific from the Red Sea and East Africa to Japan, Lord Howe Island, Hawaii, the Marquesas and Ducie Island.  Usually, they live in schools in clear lagoons and seaward reef slopes at depths that range from 20 – 490 feet (6 – 150 meters), although the are most commonly found from 32 to 450 feet (10 – 137 meters). The foods they consume as juveniles and adults are zooplankton (arrow worms, crab larvae & pelagic tunicates), some filamentous algae, benthic algae and weeds, jellyfish and hydroids.  Adults also consume organic matter such as detritus and debris.  

These fish are generally found in large schools only pairing off to spawn.  The Silver-Blotched Unicornfish (Naso hexacanthus) has been observed swimming with schools of Sleek Unicornfish in small numbers, however it is quite easy to see the differences!   The Sleek Unicornfish is on the IUCN Red List for least concern.

  • Scientific Name: Naso hexacanthus
  • Social Grouping: Groups – They break off from their large schools temporarily to spawn as pairs.
  • IUCN Red List: LC – Least Concern


The Sleek Unicornfish has an elongated body shape with a narrower caudal peduncle, features that distinguish this genus from other Acanthurids. They have a grayish to green body that is paler on the lower portion and through the anal fin. There is a yellowish brown tone just in front of the gill cover and the caudal fin is bluish, paler on the end.  Unlike other unicornfish they do not develop a protruding horn, not even a bump. However true to their name ‘Blacktongue’, they do develop a black tongue when it they reach just about 10 inches (25 cm). Juveniles are similar but with a slightly concave edge to the caudal fin.

Like all Naso species, they have the ability to quickly and dramatically change, depending on mood or environment. When trying to impress a female or indicate anger/dominance, the male will have a large radiant pale blue area on the head and neck along with blue spots and stripes on the side.

On each side of the caudal peduncle are two fixed spines or “scalpels,” however, the Sleek Unicornfish’s are fixed and weak, making them a little safer when handling.  Adult males reach 29 1/2 inches (75 cm), females closer to 20″ (50 cm) and they reach maturity at 17.7′ long.  Tangs have been known to life 30 to 45  years.

  • Size of fish – inches: 75.0 inches (190.50 cm) – Reaches 80% of adult length by age 5. Females are smaller, closer to 20″ and are mature at 17.7″
  • Lifespan: 45 years – 30 to 45 years (Choat and Axe 1996), possibly less in captivity.(Choat and Axe, 1996)

Fish Keeping Difficulty

   Once eating, Sleek Unicornfish are hardy and moderately easy to maintain as long as sufficient space is provided because they will neither thrive nor survive in too small an environment.  Obtain a specimen that is at least 4″ long as smaller ones rarely adapt to living in captivity. As they attain around 80% of their growth in the first 5 years of life, this means they will be 23.2” at that point in their lives.  They must be housed in a very large tank. It is best not to purchase small with the idea of “getting a bigger tank” later, as these fish will quickly outgrow a smaller tank. Too small of an environment can stunt their growth and they can develop ‘behavior problems.’  A healthy unicornfish will be swimming the length of the tank during the day. If you see any sulking, hiding behavior, this is an indication of stress. If their reason for sulking is not resolved or they are stressed for too long they may not recover.

   Tangs in general are susceptible to bacteria resulting from organic buildup which deteriorates water quality.  Consequently they will need vigorous filtration, protein skimming, and regular small water changes.  Going hand in hand with this, they do not produce as much skin mucus on their bodies as other fish and can be susceptible to diseases such as Marine Ich and Marine Velvet, in which a cleaner goby or cleaner shrimp can be a real asset!  Surgeonfish are definitely a candidate for quarantine when you first receive them. Sleek Unicornfish are not very susceptible to HLLE or lateral line disease as other tangs.

   As far as handling them, though the Sleek Unicornfish has a fixed, yet weak set of scalpels at their tail fin area, they still need to be captured with a container, not a net. As with all fish in this family, a cut from its scalpel can cause discoloration and swelling of the skin with a high risk of infection. The pain lasts for hours then still ends up having a dull ache.

  • Aquarium Hardiness: Very Hardy – Purchase 4″ juvenile for best results, as adults do not adapt to captivity as well.
  • Aquarist Experience Level: Intermediate – Due to required tank size.
school of Sleek Unicornfish
Image Credit: Rich Carey, Shutterstock

Foods and Feeding

The Sleek Unicornfish or Blacktongue Unicornfish are considered to be carnivores.  In the wild these fish range over large areas of steep reefs, occasionally picking at red filamentous algae, but primarily feeding on large zooplankton like crab larvae, arrow worms, jellyfish, hydroids and pelagic tunicates in the open waters.  They will eat some algae and love detritus!

In the aquarium provide various worms, live brine shrimp, mysis, larger shrimps, and euphausiids along with live marine algae (attached or free-floating) and dried algae. You can also offer chunks of prepared frozen formulas containing algae or spirulina, zooplankton, frozen brine and mysis shrimp, and flakes. Japanese Nori or other seaweed can be adhered to the aquarium glass with a vegetable clip. Feed at least 2 to 3 times a day.

Providing a vitamin supplement (including vitamin C) can help provide for their nutritional needs.  This can be done by soaking dried pellets with liquid vitamins, adding vitamins to the food, or adding a liquid vitamin into the water. It is also said that pellets soaked in garlic may help fend off Marine Ich.

  • Diet Type: Carnivore – Provide a little veggie matter too.
  • Flake Food: Yes
  • Tablet / Pellet: Yes – Soak in Vitamin C and/or garlic for better health
  • Live foods (fishes, shrimps, worms): Some of Diet
  • Vegetable Food: Some of Diet
  • Meaty Food: Most of Diet
  • Feeding Frequency: Several feedings per day

Aquarium Care

A large specimen that is constantly moving, it will spend a good deal of its time in the open water. They may jump out of an open aquarium, so be sure to have a lid. Frequent water changes are not necessary, rather normal water changes at 10% biweekly or 20% monthly are fine.

Reef tanks:
-Large Tanks 100 gallons and over, once water is aged and stable can be changed 10% bi-weekly to 20% monthly, depending on bioload.

Fish only tanks:*
-Large Tanks 100 gallons and over, once water is aged and stable can be changed 20% to 30% every 6 weeks depending on bioload.

For more information on maintaining a saltwater aquarium see: Saltwater Aquarium Basics: Maintenance. A reef tank will require specialized filtration and lighting equipment. Regular water changes done  bi-weekly will help replace the trace elements that the fish and corals use up.  

  • Water Changes: Bi-weekly

Aquarium Setup

Reaching almost 30,” the Sleek Unicornfish needs to be housed in a tank that is at least 360 gallons by the time they are 14” long, which they will reach by 3 to 3.5 years old.   In the meantime, up to 4 or 5” long they can be kept in a 100 gallon tank, then within a year, once they are 8 to 9” long, a 180 gallon tank.  A year after that, at 3 years old, they will be 11 to 13” long and ready to enter into their new 360 gallon tank.  I don’t think most aquarists want to go through the expense of changing out a tank every year, so just putting them in a 360 gallon tank in the beginning is best.  Too small of an environment can stunt their growth and they can develop ‘behavior problems’. 

All surgeonfish need an aquarium with plenty of aeration, a strong current will help to provide good oxygenation. Provide a large tank with plenty of space for swimming, especially for adult specimens, along with lots of rocks/ corals to provide some cover and for sleeping.  Have enough light to provide some algae growth, however they will be fine in a fish only tank with lower lighting as long as they get some veggie food.  The best temperatures to keep them are between 72 – 78° F; keeping in mind that the lower the temperature, the more oxygen they will have, which they need due to their high energy output!  All surgeonfish and tangs thrive with good water movement, need lots of oxygen, and love to have the water rushing over their gills. Provide strong movement in at least one area of the tank.  Your Sleek Unicornfish will spend time in the open water constantly moving back and forth along the length of the tank.  It will sleep in crevices at night, so make sure your “crevices” grow with your pet.

  • Minimum Tank Size: 360 gal (1,363 L) – 4-5″ – 100 gallons 6-9″ – 180 gallons 10-14″ – 250 gallons 14″ – 360 gallons
  • Suitable for Nano Tank: No
  • Live Rock Requirement: Typical Plus Hiding Places – Provide a cave or crevice large enough for them to wedge themselves in at night.
  • Substrate Type: Any
  • Lighting Needs: Any
  • Temperature: 72.0 to 78.0° F (22.2 to 25.6&deg C) – Lower temperatures provide the much needed oxygen for this active fish!
  • Breeding Temperature: – unknown
  • Specific gravity: 1.023-1.025 SG
  • Range ph: 8.1-8.4
  • Brackish: No
  • Water Movement: Strong
  • Water Region: All
Sleek unicornfish
Image Credit: serg_dibrova, Shutterstock

Social Behaviors

The Sleek Unicornfish, being more of a meat eater as an adult, seems to be a little less territorial over algae crops than other tangs!  They are still semi-aggressive towards other tangs, yet peaceful to most other fish.  Unless you have a huge tank (500 gallons or more), this should be the only Naso tang in the tank.   When it comes to adding other tangs or surgeonfish, it is best to make a list of the tangs and surgeonfish you want to keep, choosing tangs from different genus and different colors, different food needs, yet with similar personalities.   Buy them all as juveniles and quarantine them first, then add them all at the same time in a 500 gallon or larger tank (this size is needed for your Sleek Unicornfish to share space), to prevent aggression.  Most importantly, provide lots of caves and crevices to them to sleep in at night so there won’t be squabbling at bed time.  These “beds” need to GROW with your fish!  Though not advised, if adding a new member to an established group of tangs, changing the rock work will help alleviate too much aggression toward the “new guy”. A little chasing will occur, and nothing detrimental should happen, but if a tang is not swimming, then they are stressed and being picked on by more aggressive fish from their family.  

Sleek Unicornfish get along with most other marine fish and can be kept with a variety of tank mates, however very aggressive triggerfish should be avoided.  When it comes to peaceful fish, Assessors may feel too threatened by such a large fish and may wither away as it “hides to death.”  A large grouper can eat a smaller Sleek Unicornfish, so avoid housing them together unless the tang stays larger than the grouper at all stages of BOTH fish’s life spans.  Toadfish will also try to eat them!  Avoid pipefish, seahorses and frogfish since they are best kept separately in their own tanks. 

Although literature states that a rouge tang will nip at large-polyp stony corals, most aquarists to feed their tang properly will not run into this problem. 

Inverts can be at risk, due to their slightly different nutritional needs.  They will eat small worms, copepods and zooplankton, however larger shrimp and other crustaceans should be okay.  Keep an eye on tube worms.  On a rare occasion, an occasional tang will find the slime that clams produce quite tasty.  They may even develop a taste for clams if not well fed.  

  • Venomous: No
  • Temperament: Semi-aggressive – Peaceful toward non-tangs
  • Compatible with:
    • Same species – conspecifics: Sometimes – In tanks that are at least 500 gallons.
    • Peaceful fish (gobies, dartfish, assessors, fairy wrasses): Monitor – Assessors are too timid.
    • Semi-Aggressive (anthias, clownfish, dwarf angels): Safe
    • Safe
    • Large Semi-Aggressive (tangs, large angels, large wrasses): Monitor – Only house with other tangs if tank is over 500 gallons and all are added at the same time as juveniles.
    • Large Aggressive, Predatory (lionfish, groupers, soapfish): Monitor – As long as they are not large enough to swallow a juvenile whole.
    • Threat
    • Anemones: Safe
    • Mushroom Anemones – Corallimorphs: Safe
    • LPS corals: Monitor – Should not bother LPS if well fed.
    • SPS corals: Safe
    • Gorgonians, Sea Fans: Safe
    • Leather Corals: Safe
    • Soft Corals (xenias, tree corals): Safe
    • Star Polyps, Organ Pipe Coral: Safe
    • Zoanthids – Button Polyps, Sea Mats: Safe
    • Sponges, Tunicates: Monitor
    • Shrimps, Crabs, Snails: Monitor – Avoid very small specimens
    • Starfish: Safe
    • Feather Dusters, Bristle Worms, Flatworms: Monitor
    • Clams, Scallops, Oysters: Monitor
    • Copepods, Amphipods, Mini Brittle Stars: Monitor

Sexual differences

Like others in the Naso genus, males are larger, have a more pronounced caudal peduncle (thick area before the tail fin where the scalpel is located), and have long streamers from the top and bottom of the ends of their tail fins.

Breeding / Reproduction

The Sleek Unicornfish reaches sexual maturity at 17.7.”  Although they swim in large schools, pairs will break away to spawn along the fringes of these schools, especially during the new and full moon.  The larger males will flash their coloring while courting females, producing an intense color throughout the body.  When they are ready to spawan they rise to the surface to release their gametes.  The eggs are pelagic with an extended larval phase, which probably accounts for the vast distribution of the Sleek Unicornfish.

Some species of surgeonfish have spawned in public aquariums and there have been a few scattered reports of spawning in home aquariums, but regular spawning and the rearing of the young has not yet been reported.

For more information on breeding and the development of the fry, see: Marine Fish Breeding: Tangs.

  • Ease of Breeding: Difficult

Fish Diseases

Sleek Unicornfish produce less body slime than other saltwater fish and have been termed “dry skinned” fish by some.  This makes them very susceptible to Cryptocaryon (saltwater ich), Marine Velvet and other diseases.   The most common ailments are bacterial diseases, and parasitic infections such as protozoas, worms, etc.  Unlike other tangs, they are not as prone to Hole-in-the-Head Disease or Lateral Line Disease.

They are definitely a candidate for quarantine when you first receive them, with a stress free environment (no other fish), good quality veggie foods, places to hide and a quiet area for the aquarium.

They can be treated successfully with medical care or copper drugs, but because they have an important microfauna in their digestive system, prolonged or continuous use of a copper treatment is not advisable.   To lesson the slight possibility of Lateral Line Diseae, avoid using activated carbon, poor water condtions, and stray voltage as these ahve all been linked to this disorder.  Providing pellets soaked in vitamin C and good quality macro algae and greens will also help reverse it.   


The Sleek Unicornfish is rarely available and may often be mislabeled as a juvenile.  If available, they tend to be more expensive than other tangs. 

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