I've got a large "Copper Band Butterfly" in my 150gal FOWLR tank and he has cleared out all of the aiptasia that I had in the tank in about a weeks time but is still finding food to eat in the LR.... He remains very healthy ( two months in ) and ventures out at night to feed... That's the only time I've seen him out swimming in the tank..... I've tried feeding him brine,spectrum pellets etc but he's refused it all.... But I know he eats at night ( pods and such i reck'n )... I bought him from Marinedepot.com ( Large size )... If you buy the smaller ones on-line, they will not give you the same guarentee... However, if you find a nice one at a good reputable LFS, you may also get a good long lived one... They are still very picky eaters and one of the hardest to keep in captivity...
Bob