fresh water dips

It's just what it sounds like--you put the fish directly into FW (match temp and pH as best you can) for a prescribed time. The FW will kill external parasites on the fishes skin and gills, and the short amount of time (typically less than 10 minutes) isn't harmful to the fish in the long run.

Check a good book--most all of them describe the treatment process, and indicate what's it's good against.

I prefer not to use FW dips--they can be more traumatizing than advertised, and they don't do anything against internal or embedded parsites.
 
If you want to treat a fish for parasites, hyposalinity is probably your best bet, as there is actual proof it works every time (unlike cleaner shrimp, FW dips, and a variety of "reef safe" cures). Garlic might help the fish fight off the disease, but it isn't any guarantee. Copper based medications work if applied correctly, but they work by basically chemically burning off the fish's top layer of skin and poisoning invertebrates that can't keep them out of their system.

To do hyposalinity, put the fish in quarantine and do water changes with fresh water every 12 hours until salinity is down to 1.009. Leave the fish in water of this salinity for 4-6 weeks to kill all ich parasites. This will kill any invert in the water, but not the vast majority of fish. If you do this to every fish as soon as you get them and wait at least six weeks between introducing fish to any live rock or inverts you buy, you can permanently keep common parasites such as ich out of your tank.

Instructions on Hyposalinity:
http://www.petsforum.com/personal/trevor-jones/hyposalinity.html

Info on Ich life cycle:
http://www.petsforum.com/personal/trevor-jones/marineich.html
 
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