If you find one in your tank there are more..... They can lay eggs fomr 10-30... I thought i only had 5. when i tore the tank down total found was 15 alive and 2 that i found dead 2 days prior.
You had nymphs, and nymphs can't lay eggs because they aren't sexually mature. You were unlucky enough to get plants from someone who had an adult damselfly go "hog-wild" in their plants before shipping them off to you.
The picture in this thread is actually of a dragonfly nymph, while yours were damselfly nymphs (same insect order but different families). The dragonfly nymphs are bigger, and I'll betting the adult dragonflies don't lay as many eggs as do the damselflies. Damselflies are pretty prolific.