Fungus?

[Flu]

See just above his anal fin at the back? It looks like a bruise but I'm thinking it might be a fungus or something. Anyone have any idea to what it could be?

 

[soritan]

Does the dark area appear to be on the outside of his body, or more 'under his skin'?

Another possibility is that he's changing color a bit. Marbles tend to do that sometimes... they're like little mood rings.

 

[Flu]

Hard to tell. It's darker looking where the scales meet with each other. It doesn't look like it's under the scale.. I don't think. Kind of just looks like he was rubbed with a bit of dirt.

 

[Synirr]

I don't see anything unusual? :shrug:

 

[sukie]

Ditto

 

[Flu]

Hm.. ok. Kind of just struck me as strange to see a black marking on my fish is all.

 

[black angel]

How long have you had him and do you regularly check his water for ammonia?
It does look like he is having a colour change but it could also be melanophore migration where the pigment level darkens in response to either an ammonia or nitrite peak and this is a burn thats now healing.
It takes around two weeks for it to go back to normal so just keep an eye on it and if it spreads it could just be a colour thing.