Other medications. This is by no means a pharmacopoeia, and remember that I'm not a vet. The web's best list of fish medications is at Dr Erik Johnson's KoiVet. Don't overlook his "Medicine cabinet recommendations."
The start of knowing what you are doing, even though you're an amateur hobbyist, is to look beyond the medication's tradename and packaging, to inspect its actual ingredients. I strongly resist putting any medication -- or "conditioner" or "tonic" -- into my aquaria, unless I have a list of ingredients. Perhaps I'm missing a useful product here or there by being so persnickety, but claims of efficacy when combined with mysterious ingredients connote "snake oil" to me. And I look into what those ingredients are, and you should, too. The starting point of any search is to google that long chemical name (with its parentheses)
Three general rules about medicating fishes can save endless woes
1.Never mix medications.
2. Don't use outdated medications. Shelf lives vary. Some preparations (especially dry ones) are quite stable. Throw out those old formalin-based remedies at the back of the cabinet, if you can't remember when you bought them.
3. Don't overdose. Calculate your water volume, allowing for substrate and rocks, etc.
Levamisole. Levamisole hydrochloride, marketed as Ergamisole, is proven effective in boosting the efficiency of drugs combatting colon cancer, so it's not easy to obtain, unless you're in a rural setting and can pick it up in its alternate guise as a sheep de-wormer. A 1978 study suggested that Levamisole paralyses worms, which let go their hold and soon perish. Levamisole has been the most-recommended antihelminthic for eliminating Camallanus, but Praziquantel is effective and gentle.
Shari Sanford's excellent faq and guide on Levamisole hydrochloride tablets successfully treating worm-induced wasting sickness in Clown Loaches is archived at Loaches on Line. A well-known aquarium company markets a capsule that contains an unspecified level of Levamisole; it also has salt. Some experienced aquarists feel that there is not enough Levamisole in the commercial product for it to be effective. My former information, that if the pH is above 7.0, the drug is rendered ineffective, seems to have been a common misconception, according to Shari Sanford's informants . Her article has useful directions about dosing. Though the drug has absolutely no effect on the biological filter, carbon or polyfilters will remove it from the water.
Methylene Blue is an organic dye, not effectively anti-parasitic, but most commonly used to keep fish eggs from fungusing. It will also stain infertile eggs, which helps you to pick them out and remove them with an eye dropper. Once the eggs hatch, though, do a water change, because many fishkeepers report that methylene blue is more toxic to fry than to the eggs. Though methylene blue is not as toxic as malachite green, unlike malachite green it will devastate the biological filtration, so be sure to remove the foam sleeve from the sponge filter, or lift the biowheel from your external filter (stashing them temporarily in dechlorinated water) if you use methylene blue.
Kordon discusses methylene blue at their website.
Mebendazole and similar drugs, such as thiabendazole and flubendazole, have been the drugs of choice for treating intestinal nematodes, such as Camallanus. The drugs have a very short half-life once in solution: half an hour. So don't make up a solution in advance. The drugs work on the cellular level, preventing the nematode's formation of microtubules.
Metronidazole is active only against protozoa, especially against anaerobes, including (in humans) some anaerobic bacteria. Since it's effective against flagellates it may be effective against external flagellate parasites, like Ichthyobodo ("Costia") and Piscinoodinium. Seachem produces AquaZole, a metronidazole-based formula, and Fishy Farmacy offers metronidazole in pure crystalline form. Larry Grenier. at the Discus Resource Page, is convinced that Metronidazole is ineffective at temperatures below 90oF.
Organophosphates. These are very toxic stuff, and you won't use them lightly. The pH of the water affects the stability and toxicity of organophosphates. As with malachite green (and copper sulfate) organophosphates are even more toxic and effective when the pH is under 7.0. Their use is against multicellular external parasites, like monogenetic flukes and leeches, parasitic copepods and fishlice (Lernaea ). Don't use organophosphates against single-celled parasites, such as Ich and Oodinium. Organophosphates are highly toxic to mollusks (snails) and crustacea (copepods), but don't tell unwary friends that they are a "cure" for snails and worms and unidentified "nasty critters."
Kordon makes a trichlorfon organophosphate powder called "Trifon," which is more often used in ponds, where the margin of error is broader than in the limits of your tank. Before you make any moves, better check the Kordon site.
Fluke Tabs contain organophosphates. The active ingredient is methyl-5-benzol-benzimidazole-2-carbamate dimethyl(2,2,2-trichlor-1-hydroxyethyl) phosphonate, translatable as Mebendazole (a de-wormer) and Trichlorfon (an insecticide). Fluke Tabs are well established to be effective against trematodes (flukes), but further clinical use of Fluke Tabs, to successfully treat for Capillaria nematodes, was described in the Zebrafish Science Monitor, vol 3 no 4: it's a good description of a careful and scientific medication regime. There are further good suggestions about using Fluke Tabs from Angelswest Fish Hatchery, who warn that the effectiveness of Fluke Tabs can vary. They suggest that you begin at 50% recommended doses, watching for severe stress and be prepared to do a 50% water change.
Praziquantel. Veterinaries have been able to prescribe praziquantel as "Droncit" for some time. Now it is directly available through the Internet. Praziquantel may be supplanting organophosphates if you are treating for skin and gill flukes (trematodes). Read Dr. Erik Johnson's article "Praziquantel for Flukes". The recommended dose in the "Introduction to fish parasites" document at the U. of Florida Extension IFAS website is 2-10 ppm (mg/L) for 1 to 3 hours, in a hospital tank. But Dr Johnson finds Praziquantel so low in toxicity that you won't need to follow up with a water change, and I'd want to eliminate resistant Gyrodactylus eggs from the aquarium as they hatch. Praziquantel won't harm the biofilter bacteria.
Praziquantel resists dissolving, though you shake it vigorously with aquarium water in a closed jar for as long as you can, before pouring it into the filter outlet. I haven't ever used a half-shot of vodka to dissolve it; the white powder will eventually dissolve. If fish eat some, it won't harm them.
Dissolved organics will interfere with the medication, so a partial water change and some surface cleaning of the substrate will make the dose more effective.
Bio-encapsulation. Since the early '90s, the technique of bio-encapsulation has been used to enrich brine shrimp with essential fatty acids that are ordinarily missing in Artemia. This technique can be extended to get drugs into the intestines of fishes. Like all saltwater organisms, brine shrimp must take in quantities of water to maintain osmotic balance. (In freshwater, the osmotic situation is reversed, and freshwater fishes drink little.) So, medication for Hexamita or for Camallanus nematodes is put into the brine, and after two hours or more, when the first brine shrimp begin to die, they are rinsed off and fed to fishes. This great technique was written up in an article by Dr Beverly A. Dixon in F.A.M.A., June 1998.