Add related tags to tag page

Question

I want to add "related tags" on tag pages. So if someone visits a tag page he will see a list of tags which are often used together with the currently viewed tag. Therefor i added a query to qa_include/pages/tag.php

function get_related_tags($tag) {
    $query = "
        SELECT w2.word AS tag, COUNT(*) AS count
        FROM ^tagwords t1
        JOIN ^tagwords t2 ON t1.postid = t2.postid
        JOIN ^words w2 ON t2.wordid = w2.wordid
        JOIN ^words w1 ON t1.wordid = w1.wordid
        WHERE w1.word = $ AND w2.word != $ 
        GROUP BY w2.word
        ORDER BY count DESC
        LIMIT 10
    ";
    return qa_db_query_sub($query, $tag, $tag);
}

and added it to the title.

if (count($related_tags) > 0) {
    $qa_content['title'] .= '</h3><br>';
    foreach ($related_tags as $related_tag) {
        $qa_content['title'] .= '<a href="' . qa_path_html('tag/' . qa_html($related_tag['tag'])) . '">' . qa_html($related_tag['tag']) . ' (' . $related_tag['count'] . ')</a>, ';
    }
    $qa_content['title'] .= ' ';
}

But it seems this are not tags but all words used in questions/answers. How can i display related tags?