It appears that you have misunderstood the format of the RewriteRule. The first one is working by accident.
RewriteCond %{QUERY_STRING} page=([0-9]+)
RewriteRule ^bath-fixture-c-13.html bath-fixture/%1 [R=301,QSA,L]
RewriteRule ^bath-fixture-c-13.html bath-fixture/ [R=301,QSA,L]
The first argument to RewriteRule is the regex to match against the requested URL. The second argument is the URL to send as a Redirect to the user, so they end up at your desired URL instead. Because you want to parse the QUERY_STRING I believe you need to use a RewriteCond. If you are doing this for SEO purposes do not forget to add the [R=301].
http://httpd.apache.org/docs/current/mod/mod_rewrite.html
If you are trying to go the other direction then you need the following.
RewriteRule ^bath-fixture/$ bath-fixture-c-13.html [QSA,L]
RewriteRule ^bath-fixture/([0-9]+) bath-fixture-c-13.html?page=$1 [QSA,L]
If you can clarify your desired inputs and outputs you will get correct answers much more quickly.