Blood In Hatchet Sides

[hatchet lover]

I have 4 silver hatchets ina 30 gallon established tank, with 4 cory cats and 3 tiger bards. All hatchets doing very well but this afternoon I noticed what looked like brusing on the sides of the largerst one. 15 minutes laters both sides were red like blood from a very bad brusing, like the blood had come up to the skin and he was on top dear. Other ones are just fine. Any ideas? I have never had this to happen before. THnaks.