Well, I thought I'd show you what I use to cool my tank, coupled to this controller. It's a little 12v clip on fan, and runs at 7w. Sort of like a tiny hair dryer. It doesn't push out huge volumes of air (unlike the 10" desk fan we used previously) but the fact that it's directed precisely where a powerhead ripples the surface means that it cools very well indeed. The tank is a Percula 90 and this little fan holds it down at 26° very well. The large 10" fan blowing across the surface of the tank was virtually ineffective in comparison. So, a concentrated blast at the surface is way better than loads of air wafting somewhere over there.