You can see the two float valves underneath the fans in this picture. I don't have a sump, so they sit in the display tank. If you look in those full tank shots I posted yesterday, you can see them in the upper right corner of the tank.
The float switches are connected to a peristaltic pump that I bought from Marine Depot. The peristaltic pump is fed by a 5 gallon container of limewater underneath my tank. You could use any pump, it doesn't need to by anything special. I use a peristaltic pump b/c I don't want to dump too much limewater into the display too quickly.