Why does only one fish get sick?

Like said in the post above...

Some fish are more likely to stress out than others. The more stress the lower their immune system gets. Moving tanks is a sure way to stress out a fish. That is why the new one is the one showing symptoms. Other reasons include, bad water quality, different lighiting schedule, temp fluctuations, salinity fluctuations, ph swings. The ich is present in your tank or fish already. It generally lies dormant, or the fish in good health is able to fight it off on its own, untill stress is introduced.

Most butterflies are notorious for not being "aquarium" safe. Yes, there are definately success stories out there, but in my experience, far more death stories than success ones.. talking 90% death with most butterflies in an aquarium. To top that off, your putting it in a 40G tank? For a Klein's butterfly it lists Minimum Tank Size: 55 gallons. I would definately think 100G+ is much better than 50G for that fish. While the Klein's is one of the better ones to chose from, much better than a copperbanded, it is still a touchy type of fish that really needs its needs met for success.

Also, you have a maroon clown in the same tank.. definately a very aggressive fish, which on top of putting a butterfly in a tank too small for it which causes severe stress to the fish, it is more than likely also getting harrassed by the clown. Double whammy almost guaranteeing death to any butterfly in that tank.

Angels on the other hand.. while almost as bad as Tangs when it comes to ich, are usually hardier than butterflies depending on the type of angel (Dwarf I hope). Still, angels can also be touchy fish to get established, but once they do they usually last a long time. You may want to consider changing out the maroon clown for one a little less agressive like an ocellaris. Otherwise your going to have a hard time finding acceptable "friends" to put in the same tank.

Ah, ok then, they are great for getting rid of aiptasia and a lot people do exactly what your doing, get a butterfly to clean up the aiptasia problem and bring back to LFS for credit.

Then to answer the original question, yes, butterflies are definately more likely to get ich than most other fish. Tangs being the only ones I think show ich more. I would suggest adding a cleaner shrimp or IMO even better, a small cleaner wrasse to your tank to help get rid of any external ich showing. Keep feeding garlic soaked food as well.

Still say think of getting a different clown.. speaking from experience. I had a maroon clown for 6 years, and it is now in a new 225G aggressive reef home somewhere else. I understand the attraction, especially when juvi's, but after a year or two they really turn into bullys in the tank. Not saying what you have to do to have a successful tank, just giving you a heads up from my experience. All fish are different and you may have a very peaceful one, but I have not seen that to be the case when they turn into adults.