Comment system not working.

Can you share you comments and posts table structure;

Also there are mistakes in your get comments query:
Instead of 
       $this->db->select('*');
        $this->db->from('comments');
        $this->db->where('comments', array('slug' => $slug));
        $this->db->join('comments', 'comments.id = posts.id');
change to 
        $this->db->select('*');
        $this->db->from('comments');
        $this->db->join('comments', 'comments.id = posts.id');
        $this->db->where(array('slug' => $slug));
        $query = $this->db->get();

where clause comes after join;

and what is "posts.id" in your query?