Sorry for your loss! I can’t think of anything especially with the Apex confirming many of your tank parameters… Except, what you said in the OP about the puffer. Something, some toxin or disease killed almost every fish in an extremely short period of time. I think you’re right to consider a known source of toxin before venturing into the Terminix or bleach theory. The below post is the first thing I found about the possibility and DaddyFish reports seeing it happen. Reading to the bottom, it sounds like pufferfish can get stressed pretty easily and they don’t have to die to be lethal to other fish.
Two questions in this theory though…. Why did the puffer die? Did he get sick which caused the stress? Could it have been an ammonia spike from the mass die off? It tested zero, sure, but ammonia gets processed quickly in an established reef and you might have just missed it. Could be why he lived the longest though, only dying after the other fish were dead. Second, why weren’t inverts affected? I can’t find anything about them being susceptible to the toxin or not. They have very simple brains but they do have nervous systems that theoretically could be affected by the toxin. But would it kill them? I don’t know. Maybe if the puffer regurgitated, the other fish ate it all and the inverts weren’t even exposed. If there was ammonia I would expect some inverts to be affected but maybe there wasn’t. Maybe one fish just got sick, killed everything else, and then died. And the chromis just didn’t get any of the regurgitated food.
I wonder if
@Jay Hemdal has any thoughts on your mass die off?
Recently, I've been thinking about getting a saltwater puffer, so I started doing research. However, a web site called reefapp.com, which seems to give overall good info, said that they all (with the exception of the narrow-lined puffer) are capable of releasing a toxin out of their skin when...
www.reef2reef.com