To date, I know of no ich medication that is both effective and reef-safe. (If one ever shows up, folks - not just the companies trying to sell it to you - are going to make some _noise_ about it!) To beat ich, you'll need one of two things: copper or Chloroquine Phosphate - and you'll need them in a quarantine tank. (In fact, if your butterfly fish has ich in your display, then every fish in your display has ich, whether they're showing symptoms or not. Ich can hide inside their gills, and some fish, at least, can develop a temporary resistance to the bug, but it's still there.)
You can knock the stuff back, and run an "ich management" tank (not a great habitat for tangs, especially
Acanthurus tangs) or you can remove all of the fish to a quarantine tank, treat them for ich with something that actally kills ich, (which it does only while the theronts are free-swimming, and hunting your fish) and wait 76 days before returning them to the display.
https://www.reef2reef.com/threads/ich-eradication-vs-ich-management.188775/
While aquarists used to use pennies back in the day, they're no longer effective for several reasons. One, pennies don't dissolve in salt water all that quickly - you'd probably like to treat the fish before sometime next week. Two, copper levels in the tank would be fairly unpredictable, and... Three, pennies aren't made of copper anymore. They've got a very thin layer of copper on them for primarily aesthetic purposes, but it would take more than a penny's worth of copper to make a penny - a losing proposition for the U.S. Government!
My two cents . . . (I had to do that! (>_<))
~Bruce