Question about auto top off

[mtlion]

I have been looking into the auto top off system and this particular system:
Aquarium Maintenance & Plumbing: GAP Products Level-Loc ATO (auto top off)
states that it can be used by being "gravity fed". I was just wondering how this type of system would be set up using gravity as that would probably be the only option for be to use something like this.

[dskidmore]

My stand has three levels, so I could put a carboy or other water container up on the shelf above the tank to supply tank or sump auto top off. If you have enough room inside the stand, a shelf can hold a lower-pressure version. The house renovation alternative is to put a small hole in the ceiling for a hose, and have the gravity fed reservoir upstairs.