A siren is a Greek mythological creature traditionally depicted as having the head of a woman and the body of a bird, or just a woman with bird legs. Over time they became associated with mermaids so much that some languages only have one word for both.
Now, there are a few traditional sirens tagged as such (post #5124692 and post #881972 for instance), but most of them are just girls with a few fish features (Tearlaments notably), or just plain mermaids.
Is it useful for the tag to be disjointed like that? Personally I'd like to just remove it from all Tearlaments and similar pics and replace it with fish_girl, leaving the tag only for traditional ones (or the ones that can be deduce to be sirens from context like post #3510478).