I made a habit when I am feeding all my fish to pay attention to the way they look and behave. Sometimes it's more difficult than other times due to the types of fish I have in my tanks.
The angelfish are easy to monitor because they are large, clearly visible (they don't hide) and they are uniquely colored so I know who is who. Others, like corydoras, tend to come out at night when the tank is dark. They hide during the day so I rarely see all of them at once.
I have around 14 corydoras in my largest tanks. I call them the Merry Maids of the aquarium world, as they eat food that falls to the gravel. They do not, as sometimes believed, eat fish waste.
While I was feeding yesterday, I noticed one corydoras had a growth on its side.
My first step was to determine if this growth was contagious. I have books of fish diseases (plus the Internet provides unlimited resources) and quickly determined that the growth was most likely the result of an injury. It was not contagious, though I continued to monitor all the fish.
I did not immediately dump medicine into the tank. That can do more harm than good. It treats the perfectly healthy fish and sometimes causes side effects that makes even the healthy ones ill.
If there had been a danger of it spreading, I would have removed the fish, transferred it to a smaller tank by itself, and administered the necessary medicines there. But because corys are very hard to catch in a tank with so many decorations and plantings, I did not want to disturb the entire population in an attempt to catch it unless it was absolutely necessary.
I decided to let nature take its course. I did, however, add Immune Plus and amazon extract to bolster the fish's natural immune system, just as I had done earlier in the week with a separate tank.
This morning, I found the cory dead. I removed it promptly and examined it. The wound can be seen clearly in the picture below.
I'll continue monitoring the tank and if others begin experiencing the same symptom, I will cautiously medicate as necessary.