I have a 6-Gal bowfront tank and it does not have any type of heating. From what I have experienced the smaller your tank is the more susceptible it is to temperature swings (make sure you take a reading when the tank is at its hottest, meaning the hotter time of day etc). However, a big difference is that I live in Oakland in which the climate stays pretty temperate (rarely does it dip below 35 deg F) outside, meaning that my inside room temp. stays at a comfortable 70-72 deg F and my tank invariably has readings that reflect that. Since you're in Chicago I suppose that a heater might be a necessity during winter, but keep in mind that it does not take much to heat up that volume of water. So I would say you would be safe by only keeping your heating mat (not sure how much it heats up) during the coldest times of day to try to keep your tank temp. closer to the ideal tropical temperature of 78 deg F.