Merten's Butterflyfish (Chaetodon mertensii)

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Merten's Butterflyfish
Chaetodon mertensii
Merten's Butterflyfish (Chaetodon mertensii)
Name Merten's Butterflyfish
Name Lat. Chaetodon mertensii
Family Butterflyfishes
Family lat. Chaetodontidae
Order Surgeonfishes
Order lat. Acanthuriformes
Origin Western Pacific
Habitat Lagoons, seaward reefs
Diet Omnivore
pH 8.1-8.4
Hardness 8-10 °KH
Behavior Peaceful
Keeping Individual, pair
Reef Compatible With caution
Care Level Moderate
Life Span 5-7 years
Protection No
Metric Units
Size 13 cm
Temperature 23-28 °C
Salinity 33-36 ‰
Aquarium ~ 500 l
US Units
Size 5"
Temperature 73-82 °F
Salinity 1.020-1.025 sg
Aquarium ~ 130 gal

Distribution and habitat

The distribution range of Chaetodon mertensii is the Western Pacific, from the Ryukyu Islands to the Great Barrier Reef and the Tuamoto Archipelago. They live in deep, coral-covered lagoons and on outer reefs to great depths.


They require a well-structured aquarium with plenty of swimming space and a reef structure (hiding, resting and retreat possibilities) with living stones which they can graze on and which act like a biological filter. Only lime-rich, heavy metal-free substrates may be used as substrate

Filters, skimmers and heaters are necessary to ensure water quality, as well as pumps to simulate tides, swells and bottom currents. Lighting must match the species-appropriate day-night rhythm of the animals

Salinity: 33-36 ‰ pH value: 8.1-8.4
Carbonate hardness: 8-10 °KH Nitrate content: 2-8 mg/l
phosphate content: 0.01-0.1 mg/l nitrite content: 0.0-0.05 mg/l

For salinity, an average value should be aimed for, which may only vary slightly by +/- 0.5 ‰. Ammonia and ammonium must not be measurable. Special attention must be paid to constantly good water quality.


They are permanent eaters, feeding on small crustaceans and algae as well as coral polyps. The food change usually succeeds with stress-free acclimation without problems. The food supply should consist of a commercially available, vitamin-enriched frozen special food mix or a combination of algae (e.g. spirulina, kelp), chopped shrimp, squid and mussel meat with live and frozen food, such as mysis, krill, bosmids and artemia, as well as live cyclops, which also serve to keep them busy. It is recommended to feed small portions several times a day (3-5 times)

Regular and varied feeding promotes health and increases resistance.

Behaviour and compatibility

It is recommended to keep them in pairs. To avoid ranking fights, two different sized or two juvenile animals should be placed in the aquarium at the same time. The dominant, larger animal always develops into the male. They should only be socialized with calm, peaceful fish, as they are very susceptible to stress.

Sex dimorphism

Presumably, all butterflyfishes are sequential hermaphrodites, meaning they possess both male and female sexual expressions. External sexual characteristics are not known.

Reproduction and breeding

There are no known reports of successful breeding in the aquarium.


As reef dwellers they should not be kept in a fish-only aquarium. If kept in pairs, with sufficient activity (live stones) and varied feeding, they can be well maintained in a reef aquarium without stony coral populations (especially acropores). When socialized with too lively or aggressive fish, they sometimes stop feeding

If different species are kept together, make sure that the fish match each other in terms of water quality and temperature requirements as well as their social behavior, and that the setup meets the needs of all species kept together. New fish to be introduced must be acclimated slowly to the water in the aquarium

Further literature can be found in your pet store.


Text: Werner Knapp; Image: Franz Lowak

Source: KUITER, DEBELIUS (2007): Atlas der Meeresfische: Die Fische an den Küsten der Weltmeere, Kosmos Verlag; ENGELMANN (2005): Zootierhaltung - Tiere in menschlicher Obhut: Fische, Verlag Harri Deutsch

  • Gemäß § 21 Abs. 5 Tierschutzgesetz idgF