Thanks for the info.Not a Dragonfly nymph, it is a Mayfly nymph. Does not get as big, and eats primarily detritus and algae, unlike the predatory Dragonfly larva. The three filaments identify it as Mayfly, according to the Dragonfly Lady. Follow that link to an explanation of the difference between the two.
And you are not going to have "hundreds" show up in your aquarium. It is uncommon to see even one. They usually come in on plants, or with wild collected live foods such as Daphnia. Large fish do like to eat them, they are often used as Trout bait, and artificial trout lures are made to resemble them, among other insect larvae.