Proper quarrantine procedures are the only sure way to prevent an Ich outbreak in established aquariums. However, the presence of salt may prevent ich outbreaks and/or minimize their severity when the fish become stressed. I have attached some information from a goldfish and koi website that I have found useful. Although I have good luck with it, I am sure that salt is not 100% necessary....especially if you have live plants.
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exerpt from
www.koivet.com:
The Use of Salt as a Remedy
I recommend that you use salt, before you try anything else for parasites.
I recommend salt so darn often because it has so many benefits over other medicants. Namely:
1) It does not harm the majority of fish species.
2) It does not push sick fish "over the edge."
3) It eliminates, QUICKLY, 7 of 9 parasites I can easily recall.
4) It does not get bound out of the system by organics or sunlight.
5) It does not pose a health risk to humans contacting it.
6) It is CHEAP, CHEAP, CHEAP.
and 7) It WON'T harm your filter!
Salt knocks off the following baddies:
Ichthyophthirius
Chilodinella
Costiasis (Ichthyobodo necatrix)
Almost every single Trichodiniid/Tripartiella organism.
Glossatella
Scyphidia
Epistylis
Trichophrya
External Tetrahymena
Inhibits trematode reproduction, clears 30% of adults.
Inhibits Lerneiid reproduction