The problem with fish tank heaters is that when they break, they stick on and overheat the water. The last two fish tanks I had, the heaters broke and cooked the fish in 100 degree water.
I use a
seedling heat mat, they are about 20 bucks (new) and use only 25 watts. On average, it raises the temp of the fermenter from ~60 to 68 or so.
I completely agree with the heat wrap approach. I use the slightly more expensive fermwrap with a johnson controller set for heating. When I had a basement (60F), and I fermented without a heater, the beer would start fine, but often got too cold when fermentation slowed for the yeast to finish and clean things up. I now keep the carboy in a dorm fridge, and as fermentation slows, I start to ramp up the temperature. Say, pitch at 63, ferment at 68 until FG, and after the 6th day or so, crank to 71 for a d-rest.
Without a way to heat, you are obviously at the mercy of the ambient temp.
Good luck,
Joe