I'm not sure I follow your disc and probe, but I will explain how float switches work:
Basically, there are two types of float switches commonly used: 1) A switch that uses an actual "float" and 2) A vacuum triggered switch. With the "float" switch, you can usually set to where the float down means "on" and the float up means "off" or vice versa. So if you want a pump to stop based on a rising water level, you can set the float switch to where a rise in the float shuts-off the power. Hence an increase in water level and the pump shuts off. Many folks (including myself) use the other way. If the float goes down, a pump turns on, filling the tank or sump. When the float goes up, that pump shuts down. Many folks keeping a SW or reef tank will use this to keep water level and salinity constant. I'm setting up a FW/brackish tank that will use a float switch (the vacuum type), so I don't have to perform water top-off. This really isn't necessary in a FW tank, but since I already have the equipment, I might as well add some automation.
Vacuum switches are not as sensitve as traditional "float" switches. In other words, it takes a greater rise or fall in water level to trigger a response. However, the vacuum type doesn't stick like the "float" type. My recommendation is to use the vacuum type if you are using a doser to top-off your tank. A traditional "float" switch often works better if you are using a pump or powerhead to top-off your tank.
