with a tank that size, it can be difficult to get heaters to work properly. I usually use larger heaters than i need, and they just don't have to work as hard--but i use like a 150gal heater on a 55gal tank. if you use too large of a heater on a very small tank--like under 20gal--then it takes no time to raise the temp too high. make sure your heater is of recommended size. also, the thermostats in the heaters are horrible. i have found that submersible heaters are better, but still allow about a 3 degree fluctuation between the set temp. if the ambient temp around the aquarium changes, it will also disturb heater function. if you use metal halides on that small tank, then you may have already identified the problem. i would recommend putting the heater on a timer opposite ur light, but this can also present another problem--if you plug a heater in and take time to get it right, which can take 3-4 days to do, and unplug it/cut the power, it doesn't always keep the same adjustments when you give the power back to it. if its power is cut and regained every day by a timer, it may fluctuate even more than it does now. try it, but keep an eye on it. if u have no luck, get a new heater, and don't go cheap. get a new thermometer. good luck.