Tail is getting black

[TriniStylez]

One of my puffers tail is getting black along the end. It looks like it has been burned. Started very small and has spread up his tail. Basically, the whole tip is a thin line of black. Hes acting normal but Im worried because I have never seen this and it seems like it will keep getting bigger. Anyone have any ideas?

 

[Percula9]

Tissue is probably dying due to a bacterial infection. Remove to a sick tank and treat for seven days with Furan2. When tissue turns black it's dying.