Injured Rainbow Fish?

[Dalmen]

It looks like my Rainbow fish has been injured somehow. It looks like its throat has been cut off by something, or is this some kind of decease? I first noticed it about a week ago. It now seems that the fish is not growing as much as its siblings, but otherwise its behaviour seems normal. I added some salt to the freshwater aquarium to help the healing. Any ideas on how this happens or how to help it heal?

I´ve attached some pictures...