Your trigger isn't playing with the snails, his jaws just aren't strong enough to crush them yet.
Hmm, so you're dosing with Aragamilk? Do you test to see if you need to add anything? I suspect that Aragamilk is some sort of calcium supplement, you don't need it in a FOWLR tank since your fish aren't going to use much calcium... and dosing without testing is dangerous. You may be adding too much of something, without realizing.
If you find your nitrates are high, double the amount of water you change... with messy eaters like lionfish and triggers, I suspect you might want to do this anyhow.
Oh... Welcome to Canreef!