Heckled? Really?
People can't help you without details.
Three people asked you how long your tank has been up and running....you don't find it relevant...it is.
There are no visable parasites, spots or other signs of disease on your fish, therefore, no one can give you a positive answer as to what is going on.
You added a visably ill fish with cloudy eyes to your tank. What was wrong with her and what she died of...again, no one can tell you that without seeing that fish. ( although cloudy eyes can be caused by a bacterial infection )
You were not heckled.....you wanted advice....and were advised that your tank is too new to have that much livestock added...and that is part of your problem...whether you care to admit it or not. It is an issue.
Since you do not have a disease or parasite that is visable.....and since you have corals in your tank....dosing with anything could result in losing your corals and inverts. So, it is not advisable to experiment with different treatments just in case.
The fish in your photos don't look critically ill....no spots, no white patches, no sores, all are swimming normally. Until you can see something, it's dangerous to start dosing your tank just in case.
The fact that they appear itchy could mean something like flukes.....gaping mouths, slowing down, pale...could well be water quality....not just chemically, but airation, flow, temp, filtration, salinity.
A parasite would not explain why your corals are also not looking healthy.