I found one in my Tank also, Heres a pic of him, about the same size as yours.
I know you don't want to hear this but I suggest you get this guy out ASAP he is going to kill all your fish, they hunt them at night and all you find in the morning is half eaten bodies. The first thing he will consume is all your crabs then shrimp and lastly fish. They are very very smart and hard as hell to get out of the Aquarium, I suggest you get it right on the first attempt as he will never fall for the same trick twice.
My gut instinct tells me that you will ignore my post like the other one above that said the same thing, in that case say "Astalavista" to those fish, it's only a matter of time before he Terminates them
