A cut and paste from an article by Jorge Cortes......there has not been alot of studies regarding garlic and the effectiveness against ich. Here is part of the article....
In a parasitic protozoan infestation similar to Marine Ich, there are usually two major insults to the host fish's physiology:
1. the gross tissue damage committed by the protozoans themselves;
2. and the secondary infections that crash the party.
The applicability of such a broad-spectrum antiseptic as contained in garlic towards fending off secondary infections should be obvious, so I will set aside secondary infections at this point. Allicin has been observed to suppress the efficacy of cysteine proteinase and alcohol dehydrogenase, two tissue-demolition agents produced by another protozoan parasite, Entamoeba histolytica, (Ankri et.al., 1997), and one can easily extrapolate how garlic medication might limit the invasive and predatory damage caused directly by C. irritans.
Still, what generates as much (if not more) speculation is not the ability of garlic to restrict damage, but rather garlic's apparent ability to deliver damage to the parasites themselves -with numerous claims of outright detachment of C. irritans trophonts (the burrowing protozoans) and tomonts (the "egg cysts") as a result.
The aforementioned ability of allicin to permeate tissue and mucus enables it to invest an afflicted area thoroughly with its partially sulfurous chemical signature. The potential is there to mask the chemical cues that enable a parasite's recognition of the host, potentially confusing the invader and further suppressing the havoc it wreaks. (Indeed this would help uninfested fish dodge "Marine Ich").
Definitely, allicin brings outright chemical assault to the parasite. In one test, allicin’s cytotoxicity fell heavily against the parasitic protozoans Trypanosoma spp. and Giardia lamblia in concentrations that were well within the tolerance of 'host-tissue' fibroblasts (Lun et.al., 1994). The same penetrating power that ensures thorough investiture of the contested tissue with protective and camouflaging agents can also ensure thorough delivery of allicin's antagonism to --indeed, into-- invading parasites.
Lastly (though certainly, other properties may yet be discovered), allicin is said to reinforce the cues for cellular apoptosis, the mechanism of programmed cell-death (Thatte et.al., 2000). An innate mechanism such as apoptosis, guided with care, has the potential to severely limit the spread of say, cancer, by motivating cancerous cells to quickly self-terminate before they can multiply. If such a 'scorched earth' defense can be triggered by an infestation episode and be guided/reinforced by allicin, then yet another way may be revealed how garlic hinders both the ability of parasites to feed on host tissue and any opportunity for secondary infections to spread.
So roughly, garlic therapy can potentially
1. fend off secondary infection;
2. neutralize the chemicals used by the parasite to destroy host tissue;
3. mask host tissue, making it difficult for the parasite to recognize it;
4. deliver outright damage to the parasite.
