http://animal-world.com/encyclo/fres...n/Diseases.htm

Above is a link I have used for ID before. Sorry, I've been fortunate enough to have not needed to know much about diseases. Part of my "luck" is observing all fish (and tankmates) multiple times before purchase. Buying a sick fish carries all sorts of baggage. A "rare" fish may be such because its a poor shipper/ delicate in captivity and every purchase encourages poor hobby habits. You are also endangering the pets already under your care. I hope this fish works out for you and you choose specimens more carefully in the future.