I have kept marines for a while and have always wondered with white spot at what stage do people start treating there fish etc. I have had fish that have had it. But never lost a fish to it. I have never run the tank fallow and treated all fish in a quarantine tank. I have just made sure the fish are eating feeding garlic soaked food( I don't know if the garlic helps or not) and keep stress down ie keep hands out of tank etc. I don't want anyone to take this post the wrong way as it is something I have always wondered about.

Amo, when I first put my Regal tang in, it showed signs of ich after a few days, I just treated it with galic for a few weeks and made sure it was eating garlic soaked food and it's it made a full recovery. No signs of ich since. 

Given the scientific evidence is lacking for garlic (at least what I've read - see below), I'm reluctant to believe it works.  There are so many other parasites that look like ich, I wonder how many cases of 'ich' being cured with garlic, were actually ich?

Don't get me wrong, I feed garlic laced food too, as it definitely works for some parasites, so still worth while doing, and can't cause any harm.

Aquarium Fish: News from the Warfront with Cryptocaryon irritans Part Four of Five — Advanced Aquarist | Aquarist Magazine and Blog

Herbal Remedies (i.e. Garlic)
Using herbal remedies to combat Cryptocaryon irritans or ich has become more commonplace over the past few years with marine aquarists. Garlic, in particular, has become popular. The foods are soaked daily with garlic prior to feeding the fish for several weeks to combat ich. Unfortunately, evidence to support garlic's effectiveness or lack thereof, for the treatment of Cryptocaryon irritans is anecdotal (Bartelme, 2003a. Cortes-Jorge, 2000). It seems to work in some cases, especially with light infections, and not in others. However there is some evidence to support garlic as a treatment for some other types of parasites.

An extract of garlic, allium satiyum, has been reported to eradicate trichodinids (Madsen et al. 2000). It has also been demonstrated that garlic extract kills Ichthyophthirius multifiliis, at the theront stage within 15 hours. Ichthyophthirius multifiliis is the freshwater parasite that causes an infection that is often referred to as ick.

The extract was prepared by crushing fresh garlic cloves. The juice was then added to the aquarium at a dosage of 62.5 mg/L to kill theronts.If more than 50% of the theronts died the concentration was recorded as effective ((Buchmann, et al., 2002). A dosage of 570 mg/L was required to kill 100% of the tomocyst stage of Ichthyophthirius multifiliis. Preliminary tests using allium satiyum indicated that it has no effect on trophonts in the skin of fish (Buchmann et al., 2002). The amount of active compounds varies widely among the different varieties of garlic and the use of garlic, by adding it to the water, may prove to be unrealistic ((Buchmann, et al., 2002).

The mechanism of host location for Cryptocaryon irritans is unknown (Colorni & Burgess, 1997). It has been suggested that garlic, when administered orally to fish, may interfere with the parasites ability to parasitize by camouflaging the chemical attraction of fish (Cortes-Jorge, 2000). Chemical attraction requires further investigation (Colorni & Burgess, 1997).