I am no disase expert but this does look like red blotch disease. I gather it is common, so here is Ian Fuller's treatment report.
Treating ‘Red Blotch Condition’ in Corydoradinae
Red blotch is a condition not a disease, secondary infection can and will exacerbate the problem. The most common cause of this condition is a sudden deterioration of the water conditions, i.e. Ammonia, or more likely Nitrite spike. The condition may take 24 hours or a little longer to show in a fish, and in that time the filter system may well have over come it and the condition of the fish is spotted the water can and is often back to normal, but the damage has been done. The condition is not contagious, and each specimen within a group will have differing resistance to it.
Red blotch condition needs to be treated as soon as it is spotted, as any delay will be fatal.. 1. Large water change at least 50% if not more. If kept in a species only tank I would recommend a 75%+ water change. 2. Full dose of Melafix. 3. Add extra air stone with fairly strong flow.
It is also highly probable that this condition affects other armoured Catfishes groups, although I have neither experienced it or treated for it.
Nitrite is also responsible for what is know as "Brown blood" again not a disease but a condition, and this is the result of a high level of exposure to Nitrite, which affects the blood ability to absorb oxygen. The gills of fish suffering with this condition will be a brownish colour, instead of a nice pinkish red. Sometimes this can be fatal, but generally if good water conditions are maintained, over time the fish will recover.
Ian Fuller