I would put this in functions.php:
function main_query_mods( $query ) {
// check http://codex.wordpress.org/Conditional_Tags to play with other queries
if(!$query->is_main_query()) {
return;
}
if(is_tag()) {
$query->set('posts_per_page',15);
}
}
add_action( 'pre_get_posts', 'main_query_mods' );