What's the difference between a Siamese algae eater and a Flying fox?

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These two species are often confused. Dr Heok Hee Ng explains how to tell them apart.

The chief difference is a large fleshy flap in the corner of the mouth in the Flying fox (Epalzeorhynchos kalopterus) that’s absent in the Siamese algae eater (Crossocheilus siamensis).

Since examination of structures of mouthparts is very difficult, if not impossible with live fishes, the only other way to tell them apart are slight differences to colour pattern.

The Flying fox has a smoother edge to the black stripe running along its body — the one on the Siamese algae eater has a more ragged edge — with the area above the stripe tending to be uniformly coloured. The scales in this area of the Siamese algae eater have dark margins, imparting a net-like pattern.

The fins of the Flying fox are generally yellowish, and orange for the caudal fin, with black highlights on the dorsal, pelvic and anal fins.

Those of the Siamese algae eater tend to be clear.  

In addition, the dorsal fin of the Flying fox is more deeply notched than on the Siamese algae eater, although this may be slight.