I tried the aquarium heater route at one point and found it lacking. Fermenting in a water bath is kind of disgusting (water starts to grow stuff) and it sloshes around, etc. Its a mess. When the heater burned out (don't know if it was b/c I had it plugged into a temp control that kept it cycling or if it was a crappy unit) I went in another direction.
Now I apply heat directly to the fermenter with a reptile heating pad. You can buy the pad material and make your own, and connect it to a temp controller. A much better option in my opinion as it doesn't need a water bath and they wrap directly around the side of the fermenter.