A quick and temporary fix for having to keep turning the light on and off is to go to Walmart, Lowes, Home Depot, or another similar store and pick up a table-top lamp dimmer ($10). This will allow you to regulate how much current is going to the lamp, controlling the temp in the tank.

EDIT: Link to a dimmer from Lowes website - http://www.lowes.com/lowes/lkn?actio...-WH&lpage=none