The biggest killer of Clown Trigger, small and large, is high nitrates. Unlike a lot of other fish, and triggers, Clown Trigger can't take high nitrates and will usually die from them, as opposed to starving from not eating. You'll find one that is eating like crazy, and is nice and fat, and then all of a sudden it will die, this is usually from nitrates being very high.
As for the size, you're tank is only a 90gal, and if it's a reef tank it's going to have a good amount of rock, which cuts down on space. So, you need to be careful what size of Clown Trigger you get, as if fed correctly it will out grow the tank in no time. So you need to be prepare to move it to a large tank. In a 90gal Fowlr tank, you'll mayn't have as much LR but it still will be a problem space wise down the line.