Holygral, the tank may be new, but she has had the water tested, so it's not likely to be ammonia poisoning. Plus, the pattern of deaths doesn't fit. Some dead that should be hardy enough to cycle with, and some still alive that should drop dead from new tank syndrome. Obviously, I'd feel more confident if I could check the tank myself, but I'm thinking she has a mantis.
BTW, my reef tank cycled in a week.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef