There is not doubt that is a the Common Earwig, as it is easy to ID by the great pic and just counting the segments of the antennae = 12. It is probably a
Forficula auricularia. These are not aqautic insects and more than likey it just fell into the tank. This is not the first time I have seen a post on a terrestrial insect in a tank
Earwigs are called earwigs due to their habit of entering the "ears" of certain plants. They will and do feed off of dead material and dead animals. An those forcepst are mostly for defense pruposes.
There are only a handful of insects that can live in seawater and earwigs are not one of them. There is a "beach" earwig / "seaside" earwig common to some shores but that is not what you have, as the forceps and antennae count is all wrong and they do not live in seawater. Look for Mantis shrimp in your tank.