Actually it's more like a farmer who's never had a ####ed sheep talking about how to prevent sheep ####ing.
In other words your advice is useless because your lack of experience means you've no idea how well your own advice works. I want to talk to people who have had this problem and know specifically what it is, because this is likely to be something very specific and I want to know if it is potentially contageous.
From what I can see, anything that can kill one single fish in a matter of days is going to make diagnosis hopefully very simple and will rule out several things. The first thing it will rule out is water quality, because sudden and rapid death like that when caused by water affects all fish, especially when they are all of the same species, and usually has a rather obvious cause such as sudden pollution incidents or stocking a brand new tank or using untreated water. The second thing is it will demonstrate that the fish was probably in a very advanced stage of it's disease or viral/bacterial infection and that it can probably be traced back a long way, possibly prior to the LFS stocking them.
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