Chaetodon mertensii

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Facts
Latin name Chaetodon mertensii - Cuvier, 1831
Local name Seychelles butterflyfish
Family Chaetodontidae - Chaetodon
Origin East Indian Ocean, West Indian Ocean, Australia, The Red Sea, Indonesia
Max length 13 cm (5.1")
As aquarium fish
Minimum volume 300 l (79 gal)
Hardiness Hardy
Suitable for aquarium Suitable with care
Reef safe Not reef safe
Aggressiveness Mostly peaceful but might be aggressive towards similar species
Feed
Recommended Macroalgea (Eg. seaweed / nori)
Microalgea (Eg. spirulina)
Small crustaceans (Krill, mysis, artemia...)
Zooplankton (Cyclops, pods...)
Mostly Large polyp stone coral (LPS)
Small polyp stone coral (SPS)
Soft coral
Beware of
Eats tubeworms

This species likes to eat tubeworms.

Can nibble at clams

This species sometimes nibbles at clams including Tridacna species.

Can be aggresive

This species is not neccessarily aggresive, but it has a greater tendency towards aggresion then other species of the same genus.

Keep in mind
Requires a varied diet

This species must be fed with an appropriately varied diet.

Thrive best on their own

These fish flourish better without other members of the same species in the aquarium.

Frequent feeding

This fish requires feeding several times a day, especially when newly added.

When the fish can find its natural food in the aquarium it requires less frequent feeding. 

Well established aquarium with algae

These fish should be kept in a well run aquarium where they can "graze" algae from rocks and stones.

If there are insufficient algae on the rocks, it is important to feed more frequently and supplement with algae rich food e.g. Spirulina. 

Hiding places

This species needs good hiding places, for example, between live rocks.

Requires plenty of space for swimming.

This species revels in swimming and requires an aquarium with ample space.

Docile

This species is very shy and docile, so one should be careful when keeping it with more aggressive fish.

Eats glass anemones (Aiptasia)

This species eats glass anemones (Aiptasia).

But occasionally one finds an individual fish which refuses to eat them.

Xanthurus complex

This species belongs to what is known as Xanthurus complex, which consists of three species which resemble each other.

Chaetodon mertensii and Chaetodon madagaskariensis
These two species are very similar and is considered to be one species by some.
They are distinguished by the larger orange area than of C. Xanturus.

Chaetodon paucifascatious
This species is recognized by the area which in the other two is orange, here is reddish.

Chaetodon xanturus
Here the orange area is smaller than C. mertensii and is more moon shaped.

Descriptions and further reading
Genus description (Chaetodon)

Some species of the Chaetodon genus are grouped together in what is known as a "complex", since they are so very similar.

Regardless of resemblance, it is important to be able to distinguish them, as in some cases they vary greatly in their needs. Sometimes there are just small differences in colour or pattern, but in other instances it is vital to know where the fish originally come from.

Family description (Chaetodontidae)

The Butterflyfish are known for their attractive patterns and colours. They are closely related to Angelfishs, but can always be distinguished, as they lack the spines on each side of the head of the Angelfish.

A smaller group of these fish will seek out primairily soft corals, like Zoanthus. A larger part of the species will target different types of LPS corals. Butterflyfish are also known to seek out anemones, tubeworms and bristleworms.

Therefore it is important to choose the correct species in relation to the corals wanted, if one desires to keep Butterflyfish in a coral-aquarium.
Bristleworms, tubeworms and other small invertebrates are also a part of the diet for many Butterflyfish.

It can be problematic, with many of these species, to get them eating in the beginning, but many of the species cannot resist live zooplankton or live mussels with crushed shells. Another option is to mimic their natural behaviour by stuffing their food into coral skeletons or stones.

They ignore most other fish and are generally peaceful, therefore multiple Butterflyfish will have no problem living together. One should however be cautious about keeping similar species together unless they are a couple.

As these fish can be difficult to acclimatize and get feeding, it is important to buy healthy fish, to avoid having to deal with more problems. Make sure to check that they do not have parasites or any visible infections.

There are some species that should not be kept in an a aquarium, as they are food specialists and will almost always refuse to eat replacement foods. It can be possible to breed some species, which will eat frozen foods. Otherwise the only way to keep food specialists is by feeding them their natural diet, which consists of live SPS or LPS corals for example.

FishBase
Aquarium trade Yes
Distribution Pacific Ocean: Ryukyu Islands to the Philippines and extending to Lord Howe, Rapa and Tuamoto islands. Replaced by Chaetodon madagascariensis in the Indian Ocean, Chaetodon xanthurus in southeast Asia, and Chaetodon paucifasciatus i
English common names Merten's coralfish
Merten's butterflyfish
Mertens' butterflyfish
Atoll butterflyfish
Danish common names Atol-fanefisk
References and further reading

About references

Bob Fenner. Corallivorous Butterflyfishes… For Aquariums? - Wet Web Media - (English)

Scott W. Michael. 2004. Angelfishes and Butterflyfishes (Reef Fishes Series Book 3) TFH Publications / Microcosm Ltd. - (English)
Bob Fenner. Butterflyfishes; Separating the Good Ones and Those You Don't Want - Wet Web Media - (English)
Collection of links to additional information - Wet Web Media - (English)
Tea Yi Kai. 2014. Reef Nuggets 2: Aquatic Lepidopterans for your reef (Revised edition) - Reef Builders - (English)