+1 to above. I don't believe anyone asked, but when you introduced all these new fish, how did you do it? Did you dump the bags and all the water they came in, into the tank? Never ever dump in the water. Ever. I bought some coral off a site I won't mention, and being new and dumb, I did a full acclimation, and then dumped in the entire contents of the bag. Within like 2 days all my fish in the tank had ich, and it wasn't coincidence as I hadn't added anything else new to the tank for months before that. You never know whats in someone else's tank water, especially a LFS that recommends not to test for ammonia. My other concern is your salinity swings. They are wayyyy too much for a tank that size. Either your salinity testing tool (whatever your using), is not reading right, or your not reading it right or something is off. I don't see how it could go from .20 to .29 to .23 unless you are doing huge water changes. If you don't have a refactometer definitely get one, and definitely calibrate it to the water you use to fill the tank before you add salt. But like others have said, always QT. It absolutely sucks doing it, as like everyone else, when I buy a new fish I can't wait to see it swimming in my display tank, but the risk isn't worth the reward. Ammonia could have definitely killed the fish, but I also think you introduced something with all your new additions.