I have not seen any research that says they are dangerous if they get too large, not ordinary bristle worms at least.
If they are getting that big, perhaps there is too much food available? As far as i know, they only consume dead tissue.
I would leave them alone, and monitor the amount of detritus that is available for them to grow on.
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"It is never too late to give up your prejudices" H. D. Thoreau
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