Ok what is killing my fish. HELP

[i cant think]

Malanurus Wrasse

It’s highly unlikely he’s the suspect in this scenario. It’s more likely to have been your tangs or disease than it is the wrasse.

As for the disease side, yes this is possible and will always be possible unless everything (fish, coral, sand, inverts, rock) were all QTed. Disease can come in on any of those things which is where many QT methods end up failing in the long term and disease appears at some point - although most disease is easily combatted naturally via keeping very fat fish (feeding heavily with nutrient packed foods).