I think that you could use a much smaller heater since it's in your sump. I used a free heater that came in a bucket of salt to heat a 100+ gallon pond in my basement (which get's pretty cold) for a long time and it worked just fine. Heaters are rated based upon the fact that they will be stationary in a body of water and the water will be forced to move around in the tank in "pockets" for lack of a better description. When placed in a constant flow of water that is homogenous to the tank as a whole due to the combination of water agitation and movement, it becomes more efficient and you can get by with less of a heater. All of your water flows through your sump at one point, and since it is placed in a smaller area, it can do a more efficient job of heating that area. By placing it in your sump, you should get a more constant temperature, not one that fluctuates significantly (I would consider your fluctuations to be medium, not high). The only difference in a sump with a smaller heater is that it will probably be on more often. One reason for your 3 degree swing, now that I have written this out and had time to think about it, could be that your large heater is heating too small an area at one time and it overheats the water that passes it by. Try to point your heater in the opposite direction of the water flow. If I'm not mistaken, all heaters have the elements at the bottom and the temperature sensors in the top. In an upright setting, heated water will flow up towards the sensor, but in a flat area, the heater will have more trouble identifying the ambient temperature, especially if the hot water is being whisked away by the water flow back to the tank. The heat will not be detected again until the water travels back to the sump, which could take a while, and cause larger fluctuations. If you point it opposite the flow, with the heated water flowing past the sensor before returning to the tank, you should have less fluctuation.
