Chaetodon mertensii, Chaetodon lunula are fairly easy and could be kept together in a FOWLR 90. You could also add Chaetodon collare which is a little more difficult but still a fairly hardy BF. The key with butterfly fish is to get them from a reliable source, and to have a 100% disease free tank. Which means you should actively treat with curpamine and prazi. You should ramp your cupramine dose up to 0.4 over a week because they are sensitive to high levels of copper. Many will not take flake or pellets right away so you will need to feed a variety of frozen foods at least 2-3 times a day.