Sick Ruby tetra

[Mark Sky]

Hi all, I have a small school of ruby tetra in a comunity tank every now and yhen ones insides will begin to turn black. They seem to cary on fine apart from loosing colour but eventually die. None of my local fish shops have been able to identify the issue. They are in a 100 ltr well planted tank with no agresive fish all water perameters are fine. The tank has been up and running for about 18 months. I'll try an attatch a photo.
Thanks Mark.