here's a link to the float switch. all I had to add to this was a tub to hold the RO/DI water... a slow retern pump with hose and a lid cut to make it air tight.
I got mine from the uk but the same product can be found from the US.

Aquarium Float Switch with Suction Mounting Bracket on eBay, also, Industrial Automation Control, Electrical Test Equipment, Business, Office Industrial (end time 27-May-09 17:02:03 BST)

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Remember people the solution to pollution is dilution... weekly water changes!