Why do our marine fish just die for no apparent reason?

It’s a tough one for sure for hobbyists in the early years.

Though in saying that, many fish purchased are already dead, it just hasn’t shown yet! The stress put on them before you buy them quite often has taken them too far internally and they will die any way, no matter what you do!
If bad PH or ammonia hasn’t adversely affected their gills else where, before you or the shop, quite possibly the stress they were under or the common over abundant nitrates in wholesalers tanks set of a lengthy attack in side them by cortisol.

They have an adrenal gland that while they are under stress keeps flooding them with cortisol! After 48 hours on average this substance that is supposed to give them control over flight or fight, starts harming their organs.
The first of the fishes functions quite often to be affected are the organbs that produce and control their external immune response built into their mucus coating, thus allowing parasites to attack their flesh and on it may go till organs begin shutting down!
This can take a day to a year, but that fish will die!

For many years and still to this day, these deaths were blamed on the fish being drug caught, because the death is similar!
The other is brown blood disease from nitrites and the slower nitrate poisoning, this one will kill them as well, but quite often in a settled and spot on water environment they will repair them selves