Wondering if the tank had been overstocked – that sounds like a maybe, depending on what "plenty" means.
If so, it's likely that all of them are still recovering from their own stress. That can be a liability for them and the new fish. In contrast, if the existing fish were healthy they'd actually act as a support for the new fish.
An 8 month old tank really only needs to have a few fish in it...≤4 just to put a number on it. The rest of the time you aren't adding fish you should be adding CUC and corals....and mostly waiting and observing. If it was started with dead rock, an even slower start can be called for.
Slow stocking and long wait/observe times makes for the lowest-stress live-stocking plan, as well as being the least problematic in terms of fish or tank pests as well. Pathogens included.
If you have a micron or UV filter you can set up, that will help if there are dino's (velvet) or cilliates (ich) in the water and your fish are susceptible. "Reef safe" ich treatments can help as well in a similar way, reducing the parasite load in the water.
If you can EASILY catch the fish and give him a freshwater dip for 5-10 minutes, that can help reduce any current parasite load he's carrying. Don't cause more stress chasing him around the tank tho....only if it's an easy catch.
