Thanks for weighing in
@MnFish1 - I do really hope I am and that there is a traceable reason for these deaths that were as simple as "yes, the new fish caused half my tank to die from this specific disease."
While I've been typing the response below, I did read your response
@vetteguy53081 and do appreciate it as well. That's insightful and there is reason to believe that it could have caused issues with the dead fish.
Again, I don't think there's anything here I'm ruling out completely (perhaps with the exception of fish aggression being the cause of these deaths).
To summarize what we've speculated so far, here's the short-list:
(1) Oxygenation issue (unknown)
(2) Disease (unknown)
(3) Ammonia (lowest explanatory variable)
The reason behind why I feel disease is "unknown" in terms of explanatory power is the following, and please feel free to help clarify or articulate any additional perspectives here, because my mission here is to learn and understand. Here's where I'm stuck with regards to disease as the driving variable:
(1) I'm not sure how to interpret the fact that seemingly, extremely healthy fish (my goldhead goby, yellow tang, and foxface rabbitfish) were killed off so quickly and with nothing quite visible on their skin. Nutrition has been my #1 focus outside of maintaining stable water parameters for my corals, so these three fish have always been fed foods aimed at enhancing their immune system (i.e. algae being 99% of the diet for my yellow tang and foxface, mysis/krill/brine shrimp for goby).
An important assumption here I'm making is that the fish that died were healthy (i.e. the ones that were there before the 3 new additions). I don't think this should ever be taken at face-value, but this is what I know: they haven't shown any signs of out-of-usual behavior this week, and have always been eating throughout the meal time. Again, this doesn't mean they've BEEN healthy or ARE healthy, just an assumption that I am making based on the information / observation I've made myself.
(2) Let's assume that there was, in fact, a disease introduced by the latest additions. What are the most prevalent and suspect ones that could do this over a 36-hour period (to healthy fish, at that)? This is a genuine question - as I've stated throughout this thread, I am simply not well versed enough in the library of fish disease outside of the most obvious ones (ich, velvet, brook, flukes) -- the first three of the four are ones that I think I would have been able to observe quite clearly on the bodies of the fish I picked up.
THIS IS AN ASSUMPTION, PLEASE CORRECT ME! <-- I don't actually know if observing their lifeless skin IS the way to determine whether they died due to this disease. So the question here is: is there a disease virulent enough to spread and kill within 36 hours, regardless of how healthy the killed fish might have been? If so, which ones are they?
This is THE most important question for fish disease as the explanatory variable behind the deaths, and which would drive the most immediate action for me, as it implies a tank-fallowing protocol that I have not yet decided to execute.
(3) Assuming the QT process is questionable and resulted in a slow, internally wasting fish. Would this have been isolated to the new fish? Could this treatment have resulted in the deaths of the other fish in the tank? This is more of a direct response to your point
@vetteguy53081 - I'm eager to learn how this could have been able to spread to the other tankmates.