Chaetodon mertensii, sold variously as the Mertensii, Merten's, Orange Tail, Atoll, or Crowned Pearlyscale Butterflyfish, ranges across coral reefs from the Indo-Pacific into the Central Pacific. The body is silver to grey, overlaid with fine black lines that create a chevroned, patterned look, and a black bar trimmed in silver crosses the eye. What identifies the fish at a glance is the solid orange band covering the rear quarter to eighth of the body, unbroken by any markings. This species belongs to a complex of closely related butterflyfish separated only by the size and shape of that orange to reddish rear field, so careful comparison is worthwhile when selecting a specimen. Reported maximum size varies considerably between accounts, from roughly 5 inches to as much as 10 inches, with about 8 inches most often cited.

Adults need a mature, well-established system of at least 75 gallons, and 90 gallons or more if a bonded pair is the goal. This is an active swimmer, so open water should be balanced against generous live rock work arranged into caves and crevices that provide territory, security, and shade, with a sand substrate underneath. Water quality demands are high: pair strong biological and mechanical filtration with protein skimming, and provide moderate to high flow with good oxygenation. Newly imported fish often behave somewhat nocturnally and can be slow to settle, so subdued lighting during the introduction period helps ease the transition.

On the reef, this butterflyfish grazes coral polyps, sponges, benthic invertebrates, bristleworms and tubeworms, and takes zooplankton from the water column. In the aquarium it is best treated as an omnivore with a strong preference for meaty items. Offer a rotating menu of live, frozen, or vitamin-enriched brine shrimp, mysis shrimp, krill, copepods, chopped clam, mussel, and crab meat, and sponge-based preparations. Once the fish is eating confidently, Spirulina-based flakes or pellets and dried marine algae round out the diet.

Feed one to two times daily as a baseline. Recently introduced specimens benefit from several smaller feedings spread through the day, particularly in systems where there is little natural forage among the rockwork. Like most butterflyfish, this species may ignore prepared food during the first days in a new tank; live foods are frequently the key to triggering a feeding response, after which frozen and dry items can be introduced gradually.

Mysis Shrimp Brine Shrimp Krill Copepods Chopped Clam And Mussel Crab Meat Spirulina Flakes Dried Marine Algae
Mertensii Butterflyfish are peaceful and somewhat shy toward unrelated species, spending much of the day cruising the rockwork and retreating to cover when startled. Trouble arises only among their own kind: individuals become territorial toward conspecifics and toward similar-looking butterflyfish, so keep a single specimen or an established pair rather than a loose group. Tankmates resembling other butterflyfish are also best avoided.

Given its docile nature, this fish should share a tank with peaceful to moderately assertive companions and never with bullies that will monopolize food or harass it out of the open. It is not reef safe. Accounts document consumption of anemones, stony LPS corals, sponges, and soft corals, along with nipping at clam mantles and other invertebrates. A fish-only-with-live-rock system is the appropriate home; aquarists who want any coral at all should expect losses and limit themselves to the most robust species.

FAQ

Frequently Asked Questions

About the Mertensii Butterflyfish

  • Is the Mertensii Butterflyfish safe for a reef tank?
    No. It is known to consume anemones, stony LPS corals, sponges, and soft corals, and it will nip clam mantles, so a fish-only-with-live-rock setup is the appropriate choice.
  • What size aquarium does an adult need?
    Plan on a minimum of 75 gallons for a single specimen and 90 gallons or larger for a pair, with open swimming lanes plus live rock caves for shelter.
  • Can it be kept with other butterflyfish?
    It is peaceful toward unrelated species but turns territorial toward its own kind and lookalike butterflyfish. Keep one specimen or an established pair and avoid similar-looking species.