Your fish looks like this serpae to me:
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/www.google.co.uk/imgres?q=serpae+tetra&num=10&hl=en&biw=1088&bih=635&tbm=isch&tbnid=1F7wIo_SvOoWRM:&imgrefurl=http
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/www.tropicalfishandaquariums.com/Tetras/SerpaeTetra.asp&docid=Vk4Ao1jaW-FU1M&imgurl=http
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/www.tropicalfishandaquariums.com/Tetras/SerpaeTetra1.jpg&w=500&h=344&ei=MjaFULfyIOab1AXo14CQBQ&zoom=1&iact=hc&vpx=768&vpy=117&dur=4020&hovh=186&hovw=271&tx=248&ty=88&sig=103349512573384008980&page=2&tbnh=140&tbnw=165&start=15&ndsp=20&ved=1t:429,r:9,s:15,i:152
look at the variations..
http
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/www.google.co.uk/search?num=10&hl=en&site=imghp&tbm=isch&source=hp&biw=1088&bih=635&q=serpae+tetra&oq=serpae+tetra&gs_l=img.3..0l2j0i5l3j0i24l5.1142.8044.0.8223.16.14.2.0.0.0.222.2141.0j12j2.14.0...0.0...1ac.1.MLDeQYyO19w#hl=en&site=imghp&tbm=isch&sa=1&q=hyphessobrycon+serpae&oq=hyphessobrycon+serpae&gs_l=img.3..0i24.45695.48345.4.48886.8.3.1.4.5.0.265.445.2j0j1.3.0...0.0...1c.1.rhJloMsslck&bav=on.2,or.r_gc.r_pw.r_qf.&fp=3c7b8e1422303950&bpcl=35466521&biw=1088&bih=635
Here is a very good description really:
http
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/www.skepticalaquarist.com/hyphessobrycon-serpae
I think that settles it then
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looking at the different varieties its clear to see it is a serpae
Such an abundance of synonyms is not simply confusing: it represents the fact that the Serpae Tetras form a species flock, formed of populations with subtly distinctive coloring and, for serpae, in populations with differing chromosome numbers. Marks and blotches appear more strongly in young, healthy specimens , which may be identified by some researchers as sub-species. The various wild populations of these "Blood Characins", as German aquarists first called them, have been blurred by generations of interbreeding in captivity since these fishes first appeared in German and US aquaria in the 1920s; then they were identified both as Hyphessobrycon serpae and H. callistus. As long ago as the 1950s Gunther Sterba noted that in aquarium-bred fish, the same spawning would produce individuals with distinct shoulder-blotches together with ones in which the blotches were faint or absent, and much variation in the black of the dorsal fin.
Its basically saying the Serpaes vary hugely, both in the wilds and in captivity, the black markings are darker in younger fish than in older fish... and the black markings are present in some fish and not others. Makes sense