Fish like this have been caught recently (this Autumn) in many freshwater bodies in Sweden (like Dalbergsån) and no one knows what they are supposed to be (as written in this article). Apparently, the fish was caught on worms.

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It looks like a common roach, but the eyes are different colour and the red around the gills is uncharacteristic. Some pictures I have seen show a yellowish hue on the fins. I'd guess it is an invasive new species and probably someone out there knows what it is.

Here is a gallery with other images.

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    I guess it's more likely to be some kind of sickness (from poisoning): web-search results or see this thread for example – OddDeer Nov 8 '16 at 10:10
  • You may really be on to something. – Kenji Nov 8 '16 at 16:42
  • Environmental conditions or even some foods could play a part in these conditions to exist in fish, if it is a common roach. – Ken Graham Nov 10 '16 at 17:54
  • You guys were almost right. I just read an article where a scientist explains this case as a pigment deficiency. – Kenji Nov 24 '16 at 16:54

Well, it seems that the mystery was solved: it is a common roach with a guanin deficiency that causes them to have a different colour, as explained by this ecologist (in Swedish).

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