Hi,
Just as most reef aquarists seem to think that there is only one type of coral (see
my article in this month's
Reefkeeping Magazine), the also seem to think there is only one species of "bristle worm." Put in perspective, there are more species of so-called bristle worms than there are species of birds. Just as different species of birds do different things, different species of polychaete worms do different things. Take a look at
this article for a bit of background.
The occupy every conceivable ecological niche in the sea.
In our tanks, scavenger worms are the most common ones. See
this article for a discussion of those types, but also a discussion of a few of the predatory types that may show up from time to time.
There are, indeed, worms that will catch and eat fish. However, they will strike, kill, and swallow the WHOLE fish within a few seconds. If you saw the corpse of your fish being eaten by worms, these were the scavengers. They don't even have jaws... To get any flesh off the corpse they rub roughened patches of mouth epidermis back and forth over the dead meat. In effect, they "gum it" to pieces. This kind of animal simply can't catch or kill a live fish.