White patch on Black Tetra

[BCV2000]

Hi can anyone identify why this white patch has randomly appeared on the top one side of one of my tetras? We did a recent water change and water seems fine and all other fish are ok

 

[Rusty_Shackleford]

I'm no expert, but that looks like columnaris to me.

 

[GaryE]

I'm with @Rusty_Shackleford as far as a guess. I can say I had a similar patch appear on an African tetra a few weeks ago, and after about 3 weeks it cleared up. It was internal, not on the outside of the scales.