Nearly all wild creatures are protected in some way. Why would anyone want to flush it or kill it anyway? Damsel and dragonflies are beautiful; find somewhere to put it out 
I've grown a few of these up in a big tub in my garden. They only prey on really small food (mosquito larvae and small fry if there are any) so I wouldn't be bothered about them eating all of your fish. They can often get into your tank on plants when they're small.
How many spikes are on its tail? It looks like a damselfly nymph to me and they will have 3 spikes on their tails. They are great little creatures, mine reached about 2" long before flying (I never got to see one change into a damselfly, all I found was their skins).
If you could catch it you could put it into a water butt if you have one (as long as it doesn't have a lid) or you could put it in a pond if you have one.
cheers,
Ben![]()