Filefish may work, I have two in my 180g BUT now I cant keep many corals in the tank cause they eat them. :-(((((
I would inject the bugger again, this time be very precise, get it in the mouth. Cover the remains with more of whatever you are injecting. Make sure the water flow is off while doing this, and let it sit on the remains for at least 15 minutes before you turn the power back on.