Turn tank lights out and shine a torch on the fish to see if it looks like a gold dusting, or rusty coloured varnish.
Brown patches like velvet can be velvet.
Brown fluffy patches can be columnaris.
Here some info on velvet for you.
Oodinium (a.k.a. Velvet)
Symptoms:
Oodinium is commonly called "Velvet" because infected fish have a yellowish-brown "velvety" coating that covers its body. When the gills are affected rapid gill movements will be seen. Infected fish often scrape against objects in the aquarium in an effort to remove the parasite. In severe infestations, the skin may peel away in strips. Lack of appetite and lethargy usually accompany these other symptoms.
Cause:
The dinoflagellate Oodinium pillularis. This parasite feeds on the flesh of the fish, destroying the cells of its skin and can even infect its internal organs. This parasite is highly contagious due to the fact that it can survive up to 24 hours without a host. When without a host, they drop to the bottom of the aquarium, where they multiply and wait to find a new host.
Treatment:
Because Oodinium is extremely contagious, all fish, and the aquarium they inhabit, should be treated. Recommended medications include Paraform, Acriflavin, Quick Cure, and Formalin.
fish-disease.net