I've never had a major fish death with out it being because of me doing some thing stupid. Shock in winter with water change being too cold, forgotten chemicals, nitrogen spike.
Usually my erretmodus are my litmus test, if they get sick something is out of wacK and they are happy as and look like one may be holding.
In the last fortnight I have had 30 Tropheus die. Not from bloat or from dropsy, no distension one the abdomen or the anus. To be honest they are fancy gold fish, I just never told my wife the Caramba cost ~200/ fish. So left with the unenviable position losing the rest of my colony 12 carambras (plus 3 babies) in the following days. Usually when Ive had fish death they all die at once, not a couple a day for a week in spite of water changes.
On Mattia's recommendation I called Dr Richmond Loh and he came out on the same day, assessed the set up, the water, took swabs off the healthy fish and their poop, autopsied the dead fish and discovered that they didnt have enough body fat and possibly are underfed.
Super nice fellow and if the advice stops one ore of my tropheus dying his call out has paid for itself.