I'm not sure whether you're looking for a list or a definition...however, here's the definition as I see it:
"Hard" or "difficult" fish would be those that are either hard to get eating, or those that even tho they seem to eat well, don't live very long. Also, there are those fish that require very special parameters in terms of housing, temp, etc that most aquarists have a hard time duplicating. Many of these fish are considered "for advanced aquarists only" and others should have never been collected in the first place.
Sensitive species are those that don't tolerate poor or variable water conditions well.
A list would be too lengthy to post. However, that's where your RESEARCH comes in. Read books and articles, ask on the forums, etc.
HTH
