I would NOT, NEVER, NO WAY, trust a wireless signal (RF, X10, whatever) to control an auto top off system. If that wasn't clear ... DON'T USE WIRELESS FOR A TOP OFF SYSTEM!

In ideal conditions, the best wireles system will be 95% accurate, which means you WILL eventually have an "Off" signal that wasn't received, and you will have water on the floor, and an SG drop that may wipe out your tank!

Most top off systems use some type of float valve that use a low voltage signal. I would think you could extend that wiring far enough for your needs. If not, an EE on the board here could probably tell you how to build a circuit (or something on-line) for a line driver to make it work over an extended distance.

Oh by the way - DON'T USE WIRELESS FOR AN OUTO TOP OFF