Peppermints are one of my favorite Aiptasia removers. Of all of them, they seem to cause the fewest issues(mainly they just steal food from corals). They just take some time to feel comfortable and do their job, but the same can be said for Copperbands.
A copperband could do very well in a 50(36" x 18" x 19"). What are the other inhabitants of your tank? The biggest problem with copperbands is getting them to eat in the first place. In a reef, they are known for nipping/eating worms, zoanthids, many lps, clams on the sand bed, and will sometimes harass starfish and urchins. Not all individuals will do this and it can be curbed by feeding plenty and having well developed fauna in the tank.
If you do get a copperband (ich and velvet magnets), I like to immediately place them into hyposalinity with little acclimation(a couple hours of drip acclimation). They won't get sick, and it takes them longer to starve. Doing this, I can usually get them devouring frozen mysis and pulped oyster flesh in a few days to a week.
Personally, I'd just give the peppermints time.