Black Widow Tetra Black Markings

[martinglover]

I'm just looking at my tank and notice one of my black widow tetras has some black marks develop on its face and above the anal fin and up its back. I'm concerned this may be some form of bacteria or fungal disease? I have 4 of these fish in total however only one of them is suffering from this.

Can anyone advise if they have experienced such an issue at all with their fish? If so, how was it resolved?

Thanks,

Martin.