Where are these "blotches"? And how is the fish doing otherwise? Still eating, swiming, behaving normally?
"RED" usually means trouble, like a bacterial infection. Do you have an isolation tank setup that you can put this fish into? If not, do you have any live rock, live sand or any inverts? If all you actually have is fish in your tank, and you can't setup a separate hospital tank, you may need to treat this as a bacterial infection with a broad spectrum antibotic. If you do have to go this route, all chemical filtration will need to be terminated [like carbon filter] as well as the the UV sterializer [if you have one].