A rotting shrimp, die off from live rock. A source of ammonia. Why use damsels - all you need is the ammonia, why torture something to get there. What can be more disheartening to a beginner than to get a tank of great coloured fish, that damsels are, and to drown them in their own urine in the name of 'cycling'?
Ethically sound? I don't think so