Help me identify this insect found in fishtank
This is a Damselfly, suborder Zygoptera. Together with the Anisoptera (true dragonflies) they belong to the order Odonata, usually also named dragonflies. 

Damselflies are smaller and more slender then Dragonflies, both as larva and as adult, although there are exceptions (mostly tropical species).

It can be distinguished from mayflies by it's broad head and the broad, flat lamellae; the 3 'tails' as you call them.

Some beautiful pictures can be found here