awwww. it means your tank is healthy! and that it has a food source. are damselfly nymphs predaceous like dragonflies? are you going to rear out an adult? i'd love to get some insects in my tank (except for the tiny flies i had breeding in it for a while--they're no fun) but so many of the exciting ones are predaceous and i don't know how i'd feed them. and the small ones, of course, are fish food.
once i caught a bunch of aquatic beetles (adults) and bugs (large nymps/adults) and put them in my tank. they all were eaten or flew away except for a hydrophilid who stayed and lived off fish food for a few months. the others were mostly predators anyway and wouldn't have made it. but vegetarian diving beetles are really fun. mine was about an inch and a half long captured at a parking lot light in B-CS (the soggy part of texas for those who think it's all el paso desert).
texas has wonderful insects. even the fire ants are kinda neat.