What bug is that in my fish tank?

I give my fish tank a thoughtful clean up at the beginning of each season (every 3 months). To my surprise, during the last cleaning I found some kind of water bug hiding at the bottom of the tank. It's green. It has big yellow eyes. It has 6 legs. And it has a pair of something on its back that looks like short wings. An under water bug with wings? 

My natural curiosity forced me to start looking for answers. The quick search results yield something interesting: At any time, it's estimated that there are some 10,000,000,000,000,000,000 individual insects alive. In the US, the number of described species is approximately 91,000! I could not help but notice the word "described". Biologists find many thousands of new insect species every year! Could my water bug be a new life form?! 

Based on my further search results I got the impression that there is no easy way to use online tools to help non-scientists identify even known species of insects. Well, at least I could not tag my bug. I proceeded with a further search on some assumptions. 

How could the bug get inside of my tank? In a middle of summer (about a month ago) my kid brought  home some water plants from the Central park of Manhattan. We put those water plants in my fish tank. Most likely there were some insects eggs or even some insects on the water plants! I am not a biologist. Though, I know that insects change through stages of lifespan: egg, larva, nymph, pupa and who-knows-what-else before it become an adult. I can identify some common insects at the park: cockroaches, bugs, snails, butterflies, flies, dragonflies, grasshoppers, ants, bees and some more. 

Does my water bug have similarity with bugs known to me? I know only a couple of insects that live in water: snails, worms, some spiders, mosquitoes and that's about it. My water bug has the same big eyes as a dragonfly! The other parts of the body look different - nevertheless, those parts of the body presented. Aha! I have to take a look at dragonfly lifespan stages. 

Hooray! My water bug is a beautiful dragonfly nymph (larva)! It manage to survive in my aquarium long enough to surprise me once. It may as well surprise me by turning in to a dragonfly any time through the winter! It is something to watch out for in my apartment!
Have fun and happy bug searching :)

Click on picture to zoom