Are Corydoras Metae Delicate?

I have three corydoras metae that I bought as juveniles a few months ago from an excellent fish store. One died last night. This fish has been ailing for a while, but certainly didn't seem close to death. Her barbels have been mostly gone for a while and her fins were ragged, but she still ate and swam around. Last night I noticed that my other two meates have slightly ragged fins and now I'm worried about them. The real mystery is that they are the only ailing fish in the tank. I have six other corys, including two that are 6+ years old and four trilineatus who are doing very well. The trilineatus actually came to me with ragged dorsals that have healed completely. All of my other fish are healthy. Are corydoras metae more delicate than other corys? When I first got them they were incredibly active and extroverted and now they are slower and more shy. I thought that they had just "grown up" and weren't being puppyish anymore (like my sedate old men), but maybe they are ill, or unhealthy? They are also the only corys that swim to the surface for air regularly. I feed them a mixture of algae wafers, frozen worms and sinking pellets. My water parameters are good. The highest my ammonia has ever been is .25 on my regular water change day. Everything else has been near 0. The only thing I have been adding to the water is excel. Could they be more sensitive to excel then other fish?