I also use it on a 25g mr aqua cube that's 18x18x18. I've changed my layout (added way more plants and removed the sand) but here's what it looked like a while ago
The LFS that I go to actually recommended not using the gooseneck as it wasn't the most attractive thing out there and was kinda expensive. I ended up getting a flower pot holder and mounting that to the wall, then hanging the light down. It gives it good flexibility as you see in the picture the light was pretty high up (around 18 inches above the tank) but now as I've gotten more plants and c02 injection, I've lowered it to 8-9 inches above. At my LFS, they have 3 cubes setup and they have their lights just about 5 inches above the tank so it's really close and the plants are doing really well.