[SAT]

Given the short duration you have had many of these fish, and the way some of these species will either jump or bully each other, significant mortality might be expected. However, I agree this seems excessive.

A 72+ day fallow period generally refers to the life cycle for Ich. If you don't have Ich in the tank then that duration has no particular validity. The symptoms of Ich are pretty hard to miss, so I'm assuming that's not your problem.

This doesn't seem like a disease to me. Fish only occasionally develop a disease so quickly that you don't see any symptoms at all up to the day they disappear. Similarly, I would not suspect problems with water conditions.

A mantis shrimp would be a perfect explanation. If you have one, chances are you will periodically hear the click it makes with its claws, even if you never actually see it. The bad news is that removing a mantis shrimp can be really difficult.

Are you seeing any signs of fish being chased around? Perhaps some bruised skin, frayed tail or popeye? How about something mechanical, like an unprotected powerhead? Are you sure they are not getting sucked into your plumbing? I strong pump could pulverize a fish so it disappears.

I agree with not adding any more fish until the situation stabilizes.