The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From Neptune Systems:
AquaController Apex for the iPhone/iPod touch/iPad allows you to monitor and control your Neptune Systems' AquaController Apex, Apex Lite, or Apex Jr from the iPhone/iPod touch/iPad. Connect to your Apex from anywhere in the world and review your aquarium's current and pass water conditions using an intuitive user interface with high quality graphs and a quick overview screen for all your tanks. You can also control any of the devices connected to your Apex such as your aquarium's lights, heaters, chiller, sync the clock, initiate feed mode, and more.