it's a bit cruel in my opinion but i was having problem trying to rid my tank of a yellow tail damsel, i found the method that works best for me...would be to turn the lights on early morning preferably before the sun is up or whenever you feel that the fish in question is already sleeping...and scoop him up with a cup. i'm not sure about other fishes, but the damsel seems to wander out "blindly" when the lights are turned on in the morning.