the raccoon butterfly fish is the hardiest and can reach a size of 8 inches in an aquarium and is beautiful and also one of the cheapest.it is resistant to ink witch is a sickness that kill most butterfly fish off you cant go wrong with this pic. its 8 inch size is not a problem because of its peacful nature. the pearl scale butterfly fish is not at all hardy,and exspensive ,very small,and not as beautiful as the raccoon butterfly fish i have kept many fish i will give you a small list of exstream hardy marine fish fist the green bird wrasse,lunar wrasse,any of the hog fish,saddle wrasse,yellow tang,coral beauty angel ,fox face,sailfin tang,stare and stripes puffer,snow flake moray eel,blue damsel fish,long nose hawk fish,royal gramma,niger tigger,picasso trigger,undulate trigger ,blue jaw trigger get any of these fish and you will have instant succes, there are about ten more but half of them are in the hundreds of dalors the other half drab in appearance