These sites are not all the helpful sites out there, but the ones I have recommended most often.
Mongabay site Includes living and non-living sources of stress Also on that site, there are great fish profiles.
Swim Bladder Disease
Chart on fishpondinfo This site also has other wonderful resources.
The Freshwater Quarantine tank is an excellent article found on our site.
Medications and active ingredients
Health and Disease from Tropical Tank covers Columnaris, Dropsy, Finrot, Fungus, Gill Disease, Hole In The Head (HITH), Ich/Whitespot, Neon Tetra Disease, and Velvet (Oodinium) specifically.
Please PM me if you have a site that belongs in this list.