Copper is effective against ICH, but some fish do not tolerate the treatment well at all.
I would go with hypo-salinity (low salt content water, 1.009), as I believe it is safer for your fish.
Make your own decision after reading this;
<b><font size="2"><a href="http://www.reefkeeping.com/issues/2003-08/sp/index.php">Marine Ich/Cryptocaryon irritans</a></font></b><br>
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A discussion of this parasite and the treatment options available.