now, now....don't get all excited, this is carefully selected info designed to freak you out. (apparently it worked). the deep sea time-lapse thing you saw with eels and these isopods pointed out what they do: they are scavengers. they break down flesh as do all kinds of arthropods and more than a few vertebrates. if something is dead, they will eat it. (the opening bit is a plant. there is no reason the animal would crawl into a live fish's mouth (the fish would simply eat it) and then TURN AROUND to face out. really? hahahaa). the land version, by the way, is commonly called a pill bug, a sow bug, or, even more commonly, a roly poly, and does the same thing. it breaks down organic matter so we aren't standing around knee deep in carcasses. take a bug class. you'll learn all kindsa sh!t, like what will really gross you out or kill you.
Just to set the record straight. There IS an isopod that attaches inside the mouth of a fish. They do not eat the live fish, they eat part of whatever the fish is eating. However, they are considered a problem in some areas because they grow to the point that the fish can't eat anymore because they can't get anything past the bug. Fish starves to death then gets eaten by the bug.
Those big ones are crazy though, definitely rivaling the shark for oldest species on Earth.