The Merten’s Butterflyfish is a popular fish, yet there are two other species that look almost the same!

The Merten’s Butterflyfish Chaetodon mertensii is a handsome fish that’s often available to the aquarist. It is one of the smaller butterflyfish that only reaches about 5 inches (12.5 cm) in length. But it has a very eye catching color pattern and makes a striking addition to a community saltwater aquarium.

Its body is white with four to five vertical black chevron shaped bands on the side and the face is accented with a vertical black eyebar. There is a broad a yellow to orangish band on the rear and into dorsal and anal fin, and another on the tail fin. Other common names it is known by are Atoll Butterflyfish and Merten’s Coralfish, but its appearance has led to several more descriptive names including Yellowback Butterflyfish, Orangebar Butterflyfish, and Keel-finned Butterflyfish.

Yet even if you think you recognize this fish, that can be misleading. That’s because there are two other very similar Chaetodon butterflyfish that look a lot like it. This butterflyfish is one of three members of the “xanthurus – species complex”. These fish resemble each other so closely that they are actually identified by where they come from rather than from their looks. It’s important to know which one of these butterflyfish species you are getting. Because even though they look the same, they differ in hardiness and the types of aquariums they can be kept in.

One of the very similar “look alikes” is a difficult fish to keep. It is the Red Sea Chevron Butterflyfish (Red Pearlscale Butterflyfish) Chaetodon paucifasciatus. The other one is better known. It is the Pearlscale Butterflyfish or Philippines Chevron Butterflyfish Chaetodon xanthurus. The Pearlscale is so closely related to the Merten’s that these two will possibly hybridize in areas of where their ranges overlap. The Pearlscale and the Merten’s are very similar in  care and aquarium type as well, but the Pearlscale may be a bit safer in a reef tank with soft corals. The Merten’s is more likely to snack on soft corals and will sometimes even rid reef aquariums of the dreaded Aiptasia anemone species, such as the Glass AnemoneAiptasia pulchella.

The Merten’s Butterflyfish is moderately hardy and makes a good choice for an intermediate aquarist with some experience. It does need a good sized aquarium that is well established. A 55 gallon tank is the minimum size for a single fish, and a bigger tank will be needed if you want to keep it with other butterflyfish. Decorate the tank with rocks creating many caves for hiding places along with plenty of swimming space. It swims freely and usually spends a good deal of its time in the open water, moving in and out of crevices.

These are peaceful fish when kept in a community tank with other non-aggressive tankmates. After adaptation they usually prove to be a sturdy aquarium fish. If keeping it with more aggressive types of fish it should be added to the aquarium first. This species can be housed with some of other butterflyfishes, but it may become aggressive towards its own kind or with the other two members of the xanthurus complex.

Keeping this butterflyfish in a reef environment is a judgment call. Many reef-keepers hope to keep it in a mini reef, but like many butterflyfish it can be a coral eater. It is relatively safe with some of the more noxious soft corals as long as the reef tank is large, well stocked, and they are provided with a proper and nutritious diet. However it does eat coral polyps in nature and will most likely snack on stony corals, so keep a watchful eye.

For more Information on keeping saltwater fish see:
Marine Aquarium Basics: Guide to a Healthy Saltwater Aquarium

  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Actinopterygii
  • Order: Perciformes
  • Family: Chaetodontidae
  • Genus: Chaetodon
  • Species: mertensii
Merten’s Butterflyfish – Quick Aquarium Care
  • Aquarist Experience Level: Intermediate
  • Aquarium Hardiness: Moderately hardy
  • Minimum Tank Size: 55 gal (208 L)
  • Size of fish – inches: 4.9 inches (12.45 cm)
  • Temperament: Peaceful
  • Temperature: 74.0 to 82.0° F (23.3 to 27.8&deg C)
  • Range ph: 8.1-8.4
  • Diet Type: Omnivore
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Habitat: Distribution / Background

The Merten’s Butterflyfish Chaetodon mertensii was described by Cuvier in 1831. It has a wide range where it occurs, from Africa in the west to a chain of islands and atolls in the French Polynesia known as the Tuamotu Archipelago to the east. The range extends north to the Ryukyu Islands of Japan and south to Lord Howe and Rapa Island. This species is on the IUCN Red List as Least Concern (LC) as they have a wide distribution and there are no major threats currently identified. Other common names they are known by include Atoll Butterflyfish. Merten’s Coralfish, Yellowback Butterflyfish, Orangebar Butterflyfish, and Keel-finned Butterflyfish.

This fish is a member of the “xanthurus – species complex”. This complex is comprised of three species that are very similar in appearance. The other two are the Red Sea Chevron Butterflyfish (Red Pearlscale Butterflyfish) Chaetodon paucifasciatus the Philippines Chevron Butterflyfish (Pearlscale Butterflyfish) Chaetodon xanthurus. A fourth member once considered part of this complex is the Madagascar Butterflyfish Chaetodon madagascariensis, but it is now recognized as synonym of Chaetodon mertensii rather than as a distinct species. The C. mertensii may hybridize with the C. xanthurus.

The Merten’s Butterflyfish are found at depths between 10 – 394 feet (3 – 120 m). Their natural habitat is lagoons and seaward reefs. They dwell along the backs, the reef faces, fore-reef slopes and sometimes at drop offs where there are live coral reefs. The juveniles are seen in groups while the adults are mostly seen singly or in pairs. They feed on a variety of foods including algae, stony coral polyps, soft corals and other non-coralline invertebrates.

  • Scientific Name: Chaetodon mertensii
  • Social Grouping: Varies – Adults are seen alone or in pairs, while juveniles congregate in groups.
  • IUCN Red List: LC – Least Concern


The Merten’s Butterflyfish has a disc shaped body that is laterally compressed and somewhat elongated. It has a protruding snout tipped with a small mouth. The dorsal fin is continuous and it has a truncated tail fin. This species grows to a length of almost 5 inches (12.5 cm). The lifespan for most of the Chaetodon species is between 5 – 7 years, but sometimes longer with proper care.

The adult C. mertensii has a white body with about 4 – 5 vertical chevron shaped stripes on the side. The rear of the body and the back half of the tail fin are yellow to orangish. There are two color variants of this fish. The regular variety from the Pacific has a continuous black facial stripe running through the eye and a dark smudge-like blotch on its forehead. A second variety, found in the Indian Ocean, was previously known as the the Madagascar Butterflyfish C. madagascariensis but is now recognized as a synonym of C. mertensii. It differs from the Pacific variety as its black facial stripe is broken, being a shorter eyebar on each side. It also has a black crescent shape on the forehead rather than the dark smudge.

  • Size of fish – inches: 4.9 inches (12.45 cm)
  • Lifespan: 5 years – The average lifespan of Chaetodon species is between 5 – 7 years, and possibly longer with proper care.

Fish Keeping Difficulty

This fish is moderately hardy and is suggested for an intermediate aquarist. They will generally acclimate quickly if kept in a peaceful environment, but shouldn’t be kept with more belligerent fish or they may become timid and refuse to eat. Offering live foods may help ensure they are feeding well. After a period of adaptation they usually prove to be a sturdy aquarium fish.

  • Aquarium Hardiness: Moderately hardy
  • Aquarist Experience Level: Intermediate

Foods and Feeding

The Merten’s Butterflyfish are omnivores, in the wild they feed on algae, stony coral polyps, soft corals and other non-coralline invertebrates. In the aquarium it is important that you feed a good variety of live, frozen, and prepared formula foods with emphasis on vegetables and spirulina. These foods can include live brine, flakes, and frozen foods of all kinds including Formula I, Formula II, Angel Formula and spirulina. Japanese Nori will also be favored. Several sponge based frozen foods are now available and can also be fed to butterflyfish. Feed it at least twice a day, and if it is a tiny juvenile feed it three to four times everyday.

  • Diet Type: Omnivore
  • Flake Food: Yes
  • Tablet / Pellet: Yes
  • Live foods (fishes, shrimps, worms): Some of Diet
  • Vegetable Food: Some of Diet
  • Meaty Food: Most of Diet
  • Feeding Frequency: Several feedings per day – Offer various foods quite frequently at first. Once acclimated adults need at least 2 feedings a day and juveniles need 3 to 4.

Aquarium Care

Once adapted no special care or technique is needed to maintain this fish in the aquarium. Frequent water changes are not necessary, rather normal water changes at 10% biweekly or 20% monthly are fine. Sudden massive water changes can cause trouble.

  • Water Changes: Bi-weekly – Change 10% biweekly or 20% monthly, avoid sudden massive water changes.

Aquarium Setup

These fish need plenty of space to accommodate their size and to swim. As they can reach almost 5 inches in length, a 55 gallon tank is the minimum size for a single fish and a bigger tank will be needed if you want to keep more than one. The tank should be well decorated with rocks and/or corals with many hiding places.

In a reef environment this fish can be kept with some of the more noxious soft corals. This fish does have a reputation as a coral eater and it may take polyps of some stony coral species, so it is not strongly recommended for coral-rich reefs. The best success in a reef environment is a large tank that is well stocked and providing it with a proper and nutritious diet.

  • Minimum Tank Size: 55 gal (208 L)
  • Suitable for Nano Tank: No
  • Live Rock Requirement: Typical Plus Hiding Places
  • Substrate Type: Mix – Sand + Coral
  • Lighting Needs: Moderate – normal lighting – It can also be kept under very bright light as long as some dark areas are provided.
  • Temperature: 74.0 to 82.0° F (23.3 to 27.8&deg C)
  • Specific gravity: 1.020-1.025 SG
  • Range ph: 8.1-8.4
  • Brackish: No
  • Water Movement: Weak – Water movement is not a significant factor. It can tolerate a rather strong flow but slow-moving water is recommended.
  • Water Region: All – It swims freely and usually spends time in the open water.

Social Behaviors

The Merten’s Butterflyfish is best kept in either a large fish only (FO) or fish only with live rock (FOLR) community tank. In a reef environment it can be kept with some of the with some of the more noxious soft corals. But the tank must be large, well stocked, and they are provided with a proper and nutritious diet. A benefit in a reef is that it will sometimes help rid aquariums of the dreaded Aiptasia anemone species. However it may eat stony coral polyps, so keep a watchful eye.

These are timid fish and do best if kept with non-aggressive tankmates. Smaller non-aggressive fishes like cardinalfish, gobies, tilefish, fairy basslets, fairy and flasher wrasses are good candidates as tank mates. Larger and rather territorial angelfish like Pomacanthus and Holacanthus can be kept together with this species. Also other angelfish like members of Centropyge, Apolemichthys, Genicanthus, Chaetodontoplus and Pygoplites can be good tank mates.

If it will be kept with more aggressive types of fish, it should be added to the aquarium first. It can be kept with some of the other butterflyfish but will be aggressive with its own kind as well as other members of the xanthurus – species complex. Small but very territorial fishes like dottybacks should be avoided. Such fish as basses or scorpionfish, even if they are small enough, should also be avoided.

  • Venomous: No
  • Temperament: Peaceful
  • Compatible with:
    • Same species – conspecifics: No
    • Peaceful fish (gobies, dartfish, assessors, fairy wrasses): Safe
    • Semi-Aggressive (anthias, clownfish, dwarf angels): Monitor – If keeping it with more aggressive types of fish it should be added to the aquarium first.
    • Threat
    • Large Semi-Aggressive (tangs, large angels, large wrasses): Monitor
    • Large Aggressive, Predatory (lionfish, groupers, soapfish): Threat
    • Monitor
    • Anemones: Monitor
    • Mushroom Anemones – Corallimorphs: Monitor
    • LPS corals: Threat
    • SPS corals: Monitor
    • Gorgonians, Sea Fans: Monitor
    • Leather Corals: Monitor
    • Star Polyps, Organ Pipe Coral: Monitor
    • Zoanthids – Button Polyps, Sea Mats: Monitor
    • Sponges, Tunicates: Monitor
    • Shrimps, Crabs, Snails: Monitor
    • Starfish: Monitor
    • Feather Dusters, Bristle Worms, Flatworms: Threat
    • Clams, Scallops, Oysters: Monitor
    • Copepods, Amphipods, Mini Brittle Stars: Threat

Sex: Sexual differences

No sexual difference is noted for this species. Butterflyfish species studied up to this time indicate that these fish are gonochoristic, meaning that each fish is either a male or a female and they do not change sex.

Breeding / Reproduction

This species has not been cultivated in captivity. In the wild butterflyfish are pelagic spawners that release many tiny eggs into the planktonic water column where they float with the currents until they hatch. Once hatched the fry are in a post-larval where their body, extending from the head, is covered with large bony plates.

Marine butterflyfish have not reportedly been spawned successfully in captivity. There are however, reports of some success in rearing wild collected larvae of some of the corallivorous butterflyfish. It is hoped these captive reared fish will be adapted to accept aquarium foods, and thus broaden the species selections that can be sustained in captivity. For more information see, Marine Fish Breeding: Butterflyfish.

  • Ease of Breeding: Unknown

Fish Diseases

Many of the Chaetodon members are often very colorful and attractive to aquarists. Unfortunately some of them are rather difficult to keep for a long period. Some are exclusively coral eaters, and sometimes they suffer from “ich” (white spot disease) and other infectious diseases. Problems with disease are reduced in a well maintained aquarium. Any additions to a tank can introduce disease, so it’s advisable to properly clean or quarantine anything that you want add to an established tank prior to introduction.

Diseases that marine Butterflyfish are susceptible to include Marine Ich(white spot disease), Marine Velvet, Uronema marinum, and Lymphocystis. Some can be treated successfully with medical care or copper drugs, but some species hate sudden changes of water including PH, temperature, or any drug treatment. In the wild a cleaner wrasse (Labroides sp.) will help them by taking parasites from their bodies, however these wrasses are extremely difficult to sustain in captivity. Alternative fish such as Neon Gobies (Gobiosoma spp.) can help them by providing this cleaning service in the home aquarium.

The Merten’s Butterflyfish may be sensitive to some drugs. Be sure to observe this fish closely when medicating it, so you can remove it if it shows signs of stress. For information about saltwater fish diseases and illnesses, see Aquarium Fish Diseases and Treatments.


This fish is often available in pet stores and online and is relatively inexpensive.