The water level could be too high. If you have a chlorinator tube (where you stack the tabs), be sure it's not clogged up. They have a tendency to "weld" themselves together and not let the water flow through, which backs up the front tank. Or...The pump could be out, or you may just have a bad float.
CHL holder and Conservative...AKA "Domestic Terrorist"
I only have floats, alarms and a pump on the final tank
When my light comes on the tank level is too high, I don't have any kind of alarm on anything but the high level. when mine alarmed both times the wires from the pump had corroded through going to the float that controls the level. WW Granger has the floats. something like this: http://www.grainger.com/Grainger/items/3BY82?Pid=search
I have 2 like this , one turns the pump on , the other turns the alarm on