We i just want to start off by saying to have patience and not hate the hobby. I had the same problem with some of my chromis, i have a 55gal thats been up and running for about 3 years now perfectly fine, aside from a few fluctuations with my salinity no problems with water quality. About 3 months back i had a chromis die, all i could notice was a small mark on it like something tried to snatch at it and missed, it stayed stressed in the back of the tank for about a day and died the following night. I didnt think to much of it, just maybe he wandered to close to one of my emeralds.
I sometimes would have small pieces of corals (zoo clusters and single mushrooms) get knocked from places i had wedged them into, pretty hard too.
Then chromis #2 started acting weird, same issue, little mark, hiding in the back of the tank and died the following day.
Now i was getting upset, i had thought about the mantis shrimp but couldnt be sure because i have a pistol shrimp thats very active so i have plenty of clicks comimg from the tank. About 4 days back i was staring at the tank when i spotted him, damn little mantis, hiding in the back in the same area as my pistol, everything made sense, same time i bought a rock with some shrooms my chromis started dying, corals re-arranged and some missing emeralds as well. Hitchhiked his way into paradise and is now creating havoc, ive bought a trap but all i catch are hermits. Its a good possiblity you have a little hidden predator in your tank, just listen for clicks from the tank, which sometimes wont come if the mantis is happy and fed. But part of the hobby is understanding all the things that happen in your self contained ecosystem.