I would suggest trying a dimmer switch, as long as your heat lamp is an incandescent type that will work with them. I discovered them several years ago, and I stocked up on regular cheap 100 watt household white light bulbs for heat several years ago before they stopped manufacturing them. I now have each bulb on a dimmer switch, and I can dial them up and down if I need more or less heat, and I don't have to worry about swapping them out for lower or higher wattages, or about trying to jerry rig something to move the lamp higher or lower above the tank. It makes it really easy to adjust the temperatures when needed.
One thing to remember, however, is that not all lamps will work with dimmer switches, particularly the fluorescent tubes that are used for UV.