The heater I would recommend for a smaller tank like that is this one:
I got mine from walmart, my roommate got one very similar to it from petco.
It works perfectly! It's preset to heat the water to 78 degrees and has an internal thermometer that cuts the power off if the temperature goes above or hits 78 degrees and turns the power back on if the internal thermometer detects the water temperature has dropped below 78 degrees. Very simple and dependable product. The one I got was the 50 watt that says for 2-15 gallons. I had it in a smaller 1.5 gallon bowl and it did it's job, then I moved it to a 10 gallon tank and it still heats all of the water perfectly.
I would get the heater regardless. Your temperature may be stable now but as the weather changes the temperature may rise or fall and any fluctuation in temperature could stress out and even kill your fish!