The first link is a straight download link to the file. Unfortunately as you can see, the filename will probably change periodically during the year.
The second link takes you to a page where you must click "Validate" which installs an ActiveX control on your PC which then pokes your Windows XP/Vista/7 to see if it's a real Windows. If so, then you advance to the next page which is a straight download link.
I would make the Aquarium point to an arbitrary redirect like
http://www.serenescreen.com/getdirectx9/
and then have your web person keep that link up-to-date with whatever link is currently at Microsoft's website for DirectX 9.