One simple method is to have the light plugged into one outlet (with or without a timer), and the heater and filter plugged into a power bar with a switch, plugged into a second outlet. There are four-outlet power bars with a switch and a 3-foot cord. Switch off the power bar and do the water change, then when the tank is filled switch the power bar on. With the filter and heater in this power bar, you are not going to forget the heater as the filter running/not running is pretty obvious.