The portion of the life cycle that
Crytptocaryon irritans spends feeding on your fish host is only 5-7 days but it's life cycle can actually run up to an additional 29 days. Once the ich parasite leaves your fish it has a short free-swimming period of less than a day (this is where a UV can be effective, they are slow movers and can easily be zapped by a UV that is set correctly) and then it attaches to any hard surface (rock, glass, acrylic, snail/crab shells) and forms a sticky almost impenetrable cyst around itself where, from that single parasite, it can divide into as many as
200 new hungry ones!
Once it hatches, if it doesn't find a fish host within a few hours it will die (again, at this stage, UV's can be effective.) If there are no fish host in the aquarium, the parasites will die. However, in order to make sure you've outlasted the life cycle, you'll need to leave the tank fish-free for 4-5 weeks.
Beating ich is all about understanding the life cycle and effective treatment. FW dips can be partially effective when ich is attached as a host but often parasites that are too deeply embedded won't be killed by this method
and if the fish is heavily infested too many parasites may release at once which may cause the already weakened fish to bleed to death. Treating with copper in a QT is also hit and miss...copper will only be effective against the free swimming and host part of the life cycle. And again, if the parasites are too deeply embedded or if the fish has a heavy slime coat, the treatment may not be effective. Personally I prefer formalin dips to copper...it's just so hard to test for and maintain theraputic levels of copper in the water. Hyposalinity treatment in a QT is also very effective and there are no cumulative effects to the fish. However, it is very important to research the proper way to use hyposalinty. Do a search for ich and hyposalinity and you'll see TerryB's name come up, he knows this method of treatment better than anyone.
As far as I know, the only thing that will effectively kill the parasites in the cyst stage is a combination of formalin and Malachite green. I'll dig through my notes later and give you the exact specifics, I can't remember those off the top of my head. I also don't remember if hyposalinity will kill off the cysts or not. Time for me to do some homework
There is also antecdotal evidence that feeding the fish garlic does help prevent ich or lessen the outbreaks. However, the timing of the feedings is crucial to keep the parasites from reattaching to the fish. Do a search on Horge and garlic; he has an excellent article on the subject. I know it's posted on The Reef Tank.com archives if you're having trouble finding it.
At any rate, sorry for the long-winded post, but I hope this helps.
Alice