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.