Tank at full light. Firefish was the only one that would come out. Damsel got spooked by the high light
We do hear this issue fairly often. Determining cause of death for a group of fish is difficult to do after-the-fact. The first thing you need to do is understand that the fish losses are probably not all related. People tend to lump all unknown fish deaths into one big group, but there are often multiple causes, especially when the losses are spread across an extended time frame.
Invertebrates doing fine? That almost ALWAYS rules out problems with the water.
Fish living for weeks after arrival? As you pointed out, that rules out any issues with acclimation.
Acute fish loss is usually easier to determine - all the fish dying in a week or two points to protozoan disease in almost every case.
With flukes, the losses will be extended over weeks to months and some fish may not be affected.
IMO - the clown gobies likely had issues with the way they were collected - they live deep in coral branches, so collectors use drugs to scare them out, and this causes latent mortality, weeks later. Personally, I won't buy these any longer.
One key symptom that I didn't hear you mention is respiration rate. Many home aquarists miss that important symptom, but it can be a very good indicator of certain diseases. Any fish breathing faster than about 100 gill beats per minute may have an issue.
Jay