insect in my tank ?

What you've just described sounds just like what we called a "water
scorpion" back home in MS. I think they are some sort of dragonfly larvae or
something like that but I know they sting and it hurts (wading the creek for
many years taught me to respect the heck out of these guys). Unless you want
it for some strange reason, catch it and flush it would be my suggestion. If
it is some sort of dragonfly larvae it will catch and eat small fish.