My thoughts exactly. Fish death from disease seems to be far more prevalent in the US than in Europe. A mass dying from disease Is an unlikely event in this part of the world, especially since a fish hasn’t been added in literal months lol. Seems to be a presumptuous and unlikely cause in this case.None, because you can't see anything on a dead fish out of the water except the glare from the fish being wet.
In this hobby you have to pick and choose who you listen to. You can look at and correct the obvious issue (sulfide from rotting crap in poorly cleaned biomedia) or you can make wild speculations about lurking diseases and claims of '80% death in 8 months every time'.