I have found some very talented fish Vets. If you require a necropsy on a fish you can refrigerate them and ship them to the vet. Paracites often leave the fish immediately after death, so a fish "near " death is a better specimen for diagnosis in those cases. contact www.drjohnson.com for more online info
There is a common practices any experienced pathologist/ Parasitologist do for diagnostic of disease. This diagnostic is done preliminary by information on culture condition, current status and change, and fish stock. Depending on value of fish, you can sacrifice few fish and follow standard procedure for pathogens and parasites. For each group specific procedures need to be follow and identify the causative agents and need to do appropriate treatments. However, prevention is the best option than cure.
The American Fisheries Society-Fish Health Section Blue Book mentioned above is a good free online resource, and there is additional helpful information on the American Fisheries Society Fish Health Section website as well (www.afs-fhs.org). There is also a Fish Health Section of the Asian Fisheries Society, which can be accessed via a link on the Asian Fisheries Society (www.asianfisheriessociety.org). Some of the members of the Asian Fisheries Society Fish Health Section may be able to provide you with resources in your country.
Fish Disease: Diagnosis and Treatment By Edward J. Noga is also a very informative and great text book for these topics. Kindly refer for ur problem related.
The European Association of Fish Pathologists has published 2 nice booklets (Freshwater and Marine Edition) in the past, which are dealing with fish diseases and diagnostics. When you go to their website www.eafp.org and choose for "publishing" and subsequently "what should I do" booklets, you can download them for free.