Hi,
The cleaner shrimp won't do too much to prevent ich, but they can help to make your fish feel better and keep them from itching and scratching. Some of my fish visit the cleaner shrimp every now and then and it makes them feel better it seems. They will scrape a lot of parasites off the fish, but since ich propogates via a free-swimming phase in it's growth, the shrimp won't be able to stop outbreaks of ich and won't be able to get rid of the parasites fast enough. I still think it's worth it to have one though, they are fascinating creatures and very friendly. Mine swims up to the top of the tank and will take food right out of my hand. Pretty cool. Good luck with your shrimp!
