Natural remedies - there should be a book in this, it's complicated.

First, what does it mean? There's more than one definition.

One school has it that there are two kinds of medicine, synthetic and natural. That's pretty self explanatory, synthetic ones are ones man made that do not exist in nature. Most cancer drugs fall into this category.

This leaves the rest.

That is occurs in nature doesn't mean it's safe: mercury, arsenic, uranium, botulotoxin,anthrax, and ricin (I just set of alarms at the NSA, Hi Tommy) all occur in nature and are "natural".

Now you get things like digitals and lumbrein for heart, they're directives of the foxglove flower and earthworms respectively (Genus Lumbricus, hence "lumbrein"). So what you think isn't "natural" might be or be a man made version of something in nature. Most antibiotics are just chemicals extracted from certain molds for example.

The point is, the line between "natural" and prescription drugs may be more narrow than you think and may not even exist at all in many cases and really story of pharmacological medicine is that of taking what works form herbal, folks and homeopathic remedies over the ages. I only recently learned that homeopathy actually used to be useful, western medicine adopted all the ones that work and left the ones that don't alone. Mostly. For example: opium, TB vaccines, botox, nitroglycerine, ergot, these are all taken from homeopathy.*

The second definition of "natural" is "a substance that is already present in the body". For example, the only thing you can take that actually reduces cholesterol is niacin. Doctors now routinely prescribe it at the rate of 500 mg a day. It took 50 years to be accepted and the irony is this was only discovered as a side effect of Hoffer's work using it so successfully treat schizophrenia, depression and anxiety. The pharmaceutical industry poo-poo'd this for decades but after the statin disaster had to find something and were no longer able to ignore the glaringly obvious results that people he treated this way (that got better in 90% of cases by the way) had perfect cholesterol and adopted it about 5 years ago. Of course in their infinite wisdom they still claim he's nuts about other uses despite the fact their cure rate is 0.2% and his was 90%. This battle is still ongoing and there are a number of rather fascinating books about his.

Anyway, the point is as Will said fish get better on their own, fish, like the human body have amazing regenerative powers. The fly in the ointment here is when the immune system lacks the raw materials to function properly. In that sense the key to health is proper nutrition.

This is why you find that really successful aquarists grow their own live food and may in fact feed only live food. Disease in these tanks is virtually unknown. Because of a robust immune system. Because of the presence of the raw materials it needs.

I would even go so far s to say nutrition is more important than clean water. I've seen lots of disgustingly filthy tanks with happy fish that are breeding because they get a very high quality diet. But a clean tank with a 100% flake food diet? Good luck with that. Of course a good diet and clean water is best, but you get the point.

*I know very little about this but did find this article about the medical adoption of old homeopathic remedies, (even though these borrowings were usually unacknowledged) recently that I found rather fascinating:
http://orthomolecular.org/.../arti.../1994-v09n03-p159.shtml

Other interesting material on the subject:
http://circ.ahajournals.org/content/110/23/3512.short
http://www.thelancet.com/.../PIIS0140-6736(62.../abstract (this one is fun, Humphrey Osmond is the guy that invented the word "psychadelic". See also:
http://www.cultureunplugged.com/.../The-Psychedelic-Pioneers (this one is literally a mind blower, 56m)
https://www.youtube.com/watch?v=PH1_v0zh_gk (9m)
https://www.youtube.com/watch?v=FmTjcRf8j0I (this is for human nutrition, but is equally applicable to fish)
https://www.youtube.com/watch?v=ilUcTfPkhVY (ditto)
http://www.researchgate.net/.../0c96052b066158bca4000000.pdf
http://www.orthomolecular.org/.../pdf/2004-v19n02-p104.pdf