06-23-2013, 01:03 PM
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There are several possible causes so it needs some working through. Is it just one cory, or all of them? Are there any other marks, such as red areas? Are the gills inflamed?
Are any of the other fish showing any signs of trouble, such as increased (fast) respiration, hanging near the surface, lethargy (not active as they usually would be), loss of colour, etc?
Has anything new recently gone in the tank, be it fish, plants, wood, etc? If you have wood, is there any sign of white fungus on it?
Whenever I see any of these, I immediately do a major water change, using only a good conditioner. At least half the tank, preferably more. As long as the water parameters in the tank are close to the tap water, this will not cause any problem, and may do a world of good.