What size is the tank? Niger Triggers can get a foot in length and are flat out aggressive (also not supposed to be reef safe). Copperbands are generally peaceful and would stand very little chance of an aggressive Trigger turned on them. Copperbands are also notoriously delicate shippers and many don't survive long. There is also no guarantee they will eat Aiptasia and are a bit of a gamble with corals (generally well behaved though). As mentioned the Matted File Fish would be a better choice as it is less visible/big target for the Triggerfish and by nature is designed to specifically eat Aiptasia, but like the Copperband they are considered With Caution for Reefs.