be careful adding fish or anything for pest control
copper band butterfly's have very specific requirements and are very difficult to keep for most people. IME six lines will eat flatworms, but not aptasia. (and will go after all future additions an probably kill them) Peppermint shrimp will eat the small ones, and are probably a better option if you want that route.
Your best way is to keep an eye out - and remember to look in your sump or filtration or other areas - and kill them with the aptaisia x. it takes minutes a day, and will completely eradicate them in time. i had a really bad infestation in a frag tank, and within 3 months (of not being lazy) got them all. peppermint shrimp for me is a second choice. You can totally eradicate them as long as you dont let them go for a few weeks.