Do you think interesting a tag "Caenorhabditis elegans" ( "c.elegans" for friends :-) ? It seems not available and my reputation is far from the necessary to create it.
I am in general not a fan of tags for particular model organisms.
From my perspective, the point of model organisms is that they are models - the relevant tags are to the biology being studied. Except for some very narrow cases relevant to the model organism itself (in which case the model-organism tag is appropriate), few people 'care' about C. elegans specifically, they are just using that organism as a tool to infer something about biology more broadly. Development, molecular biology, neural circuits are all possible tags that are more useful.