dead snail

I’ve got 4 nassarius and about 4 bumblebee snails, I’ve tried to introduce 1 trochus snail at 2 separate occasions now they’re dead and being eaten by the next day. Any idea why the trochus would keep dying but not the others?

How do i prevent this from happening to the other corals in this 75 g tank? I posted a pic of parameters and some of the corals. I had high phosphates but now it has been maintained. A green chromis, anemone, and a turbo snail had skunked and died in this about a month and a half ago and i had...