Help Needed On Dead Fish

[axl]

Hi Guys,
come home from work today to find this fish dead in the corner with the only sign of somthing wrong last nite when it started to hide in the corner late last nite. I have another that has been hiding in the corner for two days now, all these fish have full colour still and no sign of illness. What is this disease???. The pic shows black lines through the fish which i have never seen before. What to do. I havent had a real good day. All the ammonia, nitrates etc are fine and im runnning a 15w uv sterilsier