Golden Loach Getting Black Marks On Its Skin

[JeffRoyle]

Hi, I have just joined as one of my two Golden Loaches is getting dark marks on its skin.  It is on the outside and has progressed rapidly over the last fortnight. They are both about four years old and have lived happily together, both lived under the same rock.  However, about a month ago the larger one moved out of the rock and has stayed on the other side of the tank.  He then started developing these horrible looking marks.  Does anyone know how I should treat it?

 

[fluttermoth]

I'm afraid sucking loaches become very aggressive as they mature, so even though they've been okay up to now, you're going to have to look at separating them.
 
As for the black marks, that could have many causes... can you give us some more details of your set up, please? Tank size, temperature, filtration, water changes, any other fish, results of any water tests you've done, would all help.