Since I've never had a damselfly over my tank, I will have to figure out a way of releasing it to the outdoor so that it will have a good life. May be I will just let the tank cover stays open overnight and let it flies away at its own choosing.
Update: tank light just went off and the little guy flew out to the balcony.
Hi Joycould, thanks for the cautionary note on damselfly nymphs. I too believe that the damselfly had came from the tank (the room's balcony windows and doors are fitted with insect screen). I suspect it all started with a tiny water hyacinth that I picked up from a farmers market couple months ago. At the time, the tank had some algae. After introducing the water hyacinth, all algaes were gone and the water hyacinth grew to produce several off-shoots.