ich

[fkshiu]

I highly doubt that did anything. If you examined the fish under a microscope you'd probably find parasites.

More likely, what happened his that the parasites simply went into a different part of their life cycle by dropping off the fish into the substrate during darkness. From Steven Pro's article:

http://www.reefkeeping.com/issues/2003-08/sp/index.php

"Mature trophonts leave the host and tomites exit the theront/cyst in the dark (Yoshinaga & Dickerson, 1994). Imagine if you will, a fish that randomly acquires a single Ich parasite. After a couple of days when the trophont is well fed, it prepares to drop off its host but waits for the environmental trigger of darkness. Meanwhile, the fish prepares to "bed down" in its favorite hiding spot in the aquarium; the same fish occupy the same spot practically every night. Now, the trophont leaves the fish, encysts, and begins to multiply. Several days to weeks go by and that same fish returns to its same spot at night, only this time there are hundreds of infectious theronts seeking out a host/victim in the same area. I am sure some of you are thinking that this is absolutely diabolical. Others can appreciate the simple beauty of this plan. To me, it is just another reminder of how remarkable evolution and adaptation is."

If ich were this easy to kill natural selection would have wiped it out eons ago.