how to add # before tags?

You could just do cosmetic changes on your theme with some javascript:

<script type="text/javascript">
jQuery(document).ready(function(){
    jQuery('.tags li a').prepend('#');
});
</script>

<ul class="tags">
    <li><a href="https://wordpress.stackexchange.com/tag/sometag/">sometag</a></li>
</ul>

error code: 523