Treating fish for Diseases and Parasites: "an ounce of prevention is worth a pound of cure."
First of all, I've assembled links to some dependable sources of information about fish diseases under the heading "Ich on the Web", because I figured panicky fingers would click to Ich.
Second, don't forget as you look around the pages that follow in this section, that I'm a fishkeeper, not a diploma-carrying veterinarian. I'm more of a fishkeeping journalist than a scientist. If you want the best, first-hand information, and if you have the patience to read about diseases and parasites and their treatment, I'd recommend you go to websites maintained by schools of veterinary medicine or aimed at aquaculture facilities ("fish farmers"), rather than to hobby-oriented websites maintained by the purveyors of bottled cures. Avoid "dumbed-down" information and the well-meaning hearsay you may get at on-line fish forums.
The web expression "YMMV" ("Your Mileage May Vary") reminds you that conditions of warmth or pH or dissolved organics can have crucial effects on the toxicity and effectiveness of medications from one tank to another. Furthermore, not all techniques suited to outdoor ponds or aquaculture facilities will be appropriate for the indoor aquarium, naturally. And conversely some rigorous strictures about medicating fish that will eventually become food don't apply to strictly ornamental fish.
Use caution with any medication. An early visitor here took me to task for not emphasizing strongly enough the dangers of cavalier mishandling of toxic substances, such as malachite green or formalin. Don't spill toxins on food preparation surfaces or your skin. Always scrub thoroughly after medicating, even if you didn't apparently contaminate yourself. The fish cupboard may need a child-proof lock, especially where undisciplined children roam unsupervised. My critic said something that can serve as a useful maxim: "Start with the least dangerous methods." If a salt bath will cure an infestation of minor ciliate skin parasites, you may be able to avoid more toxic malachite green/formalin, for example. Organophosphates are even more toxic; if you still use them at all rather than Praziquantel, save them for multicellular parasites.
Calculating dosages. Dosages are often expressed as parts per million ("ppm"). One part per million is the equivalent of one milligram dry weight in 1000 milliliters (i.e. one liter) of water. This is the same as 3.8 milligrams per gallon, since one gallon equals 3.8 liters. If you're using dry powder, check the label to see what percentage of the powder consists of "inert carriers" and adjust accordingly.