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"Chinese banner" diptych, 2 panels of 40x50cm (total length 1m.)

In 2011 a new species of Chaetodontidae, Heniochus diphreutes, was found by comparative analysis of the morphological characteristics in a taxonomic revision of the family Chaetodontidae in China. Have a look at the elongated dorsalfin and you will know why their popular name is Bannerfish. 
The species has been imported many times as H. acuminatus. Interesting for marine aquarists as H. diphreutes is considered 'reef safe' while H. acuminatus is known to nibble on corals. So, to avoid your collection of live corals being destroyed by your Bannerfish, the question is, how can the two species be distinguished? 

Heniochus diphreutes (left) and H. acuminatus (right)

Well, if you like details you may consider to do some counting. H. diphreutes usually has 12 spines in its dorsal fin and 2-3 rows of teeth on both jaws (versus 11 spines and 5-7 rows of teeth in H. acuminatus). As Bannerfish are active swimmers I reckon counting is not the easiest way. Luckily there are more distinguishable, external characters available. The ventral profile of the head is convex in H. diphreutes (versus almost straight); the anal fin more angular, and the black area on the posterior part of the anal fin usually extending anteriorly to the longest soft ray (versus more round, and usually not extending anteriorly to longest soft ray in H. acuminatus). Probably the easiest way to tell the two species apart is the length of the snout, which is considerably shorter than the eye in H. diphreutes. 

Want to know more about these fish? Here's the full description:

Zuo, Xia-Yan & Wen-Qiao (2011): "A new record species, Heniochus diphreutes, Chaetodontidae from China"- Zoological Research, Vol.32, No. 3: 349-352.

Have fun reading it...it's in Chinese.