Health and diseases
Health management is more effective than trying to cure disease. You already know the prerequisites for keeping captive fishes disease-free over the long term. They are all based on protecting fishes from disease vectors introduced from outside your system, from toxins like ammonia and nitrite produced within the system and from the effects of stress.
It's well established now that the pre-conditions for viral infections, bacterial disease, and even for successful infestations of some parasites, are all associated with long-term sub-clinical stress, which can be produced by crowded conditions (all aquaria present unnaturally close contact with aggressive or diseased fish), or by contaminated water (all aquaria are eutrophic compared to uncontaminated natural waters) and tend to result in compromised immune systems. Think of the parallels to human populations, where 1. balanced nutrition, 2. proper housing, 3. water and sewage treatment and 4. epidemic controls have extended more fruitful lives than any advanced medical intervention. Those four human factors apply directly to the aquarium.
General links. These sites offer some of the widest overview of fish diseases. You'll find some more specific links at the bottoms of the pages within this folder that discuss the various parasites.
University of Florida Institute of Food and Agricultural Sciences (I.F.A.S.) has many documents covering parasites of fish and other issues that concern Florida's fish farmers and aquaculturists. In one RuthEllen Klinger and Ruth Francis-Floyd offer an "Introduction to freshwater fish parasites." But look through the other aquaculture-related topics at the University of Florida Extension Service's EDIS, the Electronic Data Information Source, of which this document is just a part. The University of Florida's Tropical Aquaculture Laboratory has many articles concerning aquaculture relevant to aquarium systems, such as its series of articles covering parasites of freshwater fishes.
FishDoc is a wide-ranging site covering diseases diagnosis and treatment, even a section "Using your Microscope."
At Koivet Dr Erik Johnson, DVM, emphasizes articles on diseases and medications at his long-established website, with wider applications than just to koi.
You'll find excellent details on medications, symptoms, quarantine in Dionigi Maladorno's thorough article, "Symptom-based treatment of common Discus diseases", 1998, which was originally posted at AquaWorld e-magazine.
Drs Foster & Smith are a dependable source for medications purchased on-line and for articles about parasites and other maintenance problems.