It's a damselfly nymph. If you have small shrimps, it would be a good idea to catch and remove the damselfly nymphs, as they are fierce predators.
The 'tails' are actually the gills, which absorb oxygen from the water. When there is low dissolved oxygen, the nymphs are known to wag their abdomens, in a bid to create some current and aeration that will raise oxygen levels.
See my post here in another thread about my attempts to raise damselfly nymphs to adulthood. You could try the same, and take photos of the adult that has emerged. But it's best to keep it in its own container, to be fed with live prey like tubifex worms and adult brine shrimp.