I'm sorry to hear about the loss of your fish. I agree with all of the replies above.
If you don't have a quarantine system and you want more piece of mind you can use a service like Dr Reef's (google: Dr Reef's Quarantined Fish). Dr Reef has a lot of vendor feedback on this forum if you want to do your due diligence. They'll acquire your fish and quarantine/observe them for you. They post their procedure on the web site--it's fairly thorough. No quarantine is 100% effective and there's still shipping stress and acclimation but it's cheap insurance if you don't have a QT tank.
For aggression you can purchase an acrylic isolation box to put in the tank so the other fish get acquainted with their new tank mate. That's also not 100% effective, and harder to do with large fish, but it's an extra precaution.