Well i have countless experiences with mean fishes, both fresh/saltwater. A 2-3 week isolation might do the trick sometimes. But for me giving away/up either 1 of em is usually the ultimate solution 4 me... It all depends on ur luck anyways.
Had a cute lil', innocent lookin' 1.5" target puffer who snapped a fin/tail or 2 of some larger fishes. Din't think it would do it but I saw it ate live shrimps & it cleanly snapped & chomped up the shrimps like scissors cutting up papers !! Really, really, really sharp teeth... So i isolated it & kept it 4 a while b4 i gave it away. So i guess ur puffer was most probably the culprit..
Had an aggresive domino damsel who stressed killed 2 lil' percula clowns. Then I added a lil' clarkii clown which was being chased senseless by the domino damsel!... Isolated it but failed. It was then i decided to give away the domino damsel if I were to add any new fishes into the tank in future. It took 2 person, 3 nets & almost an hour to net the bully bcoz of all the rocks & hidding places!
Used to have a boxer shrimp about 3"x3" & lost 1 leg/pincer a day to a hippo tang & finally limbless & obviously died... I tot they're supposed to be symbiotic in a way? Me & especially the shrimp didn't saw that one coming...
I used to hv 4 Discus's: A,BC & D... A was always harassing B, C & D... B was harrassing C & only D seems to be the docile one (It eat's from my fingers & allows u to touch him as well)... Tried isolating all of em 4 a few weeks but it didn't work. In the end B & C got killed, gave away A(big bully) & kept D for almost 2 years now & lives harmoniously with some guppies, neon tetras, bronze catfishes & a lil' pleco.