05-24-2013, 09:50 AM
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As Eric mentioned, the first thing to do when you see a fish with an obvious problem is to test the water for ammonia, nitrite and nitrate. I also test the pH, as this can cause issues should it suddenly alter.
If it is just one fish, and the others (all species but especially the same like other corys) appear normal, I would be inclined to just leave things, unless there are very clear symptoms (which is not usually the case). If other fish are showing problems, the next step is usually a water change, 1/2 to 2/3 of the tank volume. This can do no harm (assuming parameters of tap water are close to the tank), and often solves the issue.
But there are cases where a fish has been injured previously--during initial capture, at the fish farm hatchery, in transport, in the store, etc--and there is nothing you can do in such cases as internal damage caused by physical injury or stress is undetectable and often non-treatable. Such fish usually die.
The red mouth area and lack of barbels might be suggestive of other issues, though not likely to cause the fish's demise unless there was an infection. Corys can live fine without barbels, I have had a few over the years, but the redness is not normal. Rough substrates can be detrimental; what is the substrate in this tank? We can consider this just in case, to avoid future issues.