Auger seems to be the best option, and I can tell you because I've spent two years now looking at and for really well done automatic water change systems. There aren't a lot out there yet.
If you want some other ideas there have been people who have automated the process using their Reef Keeper controller on the Digitial Aquatics forum. Even if you don't have an RKE, some of the styles and problem solving techniques might be worth it for you.
*Edit* While I was looking for something else this thread on RC popped up about different mixing stations. I know they aren't all automated or anything, but it might give you some more ideas.
Salt Water Mixing Stations Let's See Them