Just a thought... I'm not sure if anyone has used a temperature controller for fans... not that they can drop the temperature that much, but if set to switch on when the temp is warm and off when the temp is cool, it may help reduce cost and keep the fishes happy?
Anyways, a commercial auto top-off system is not that much more expensive compared to a DIY (unless you really have lobang). Of course, if DIY is your thing, go for it. Satisfaction of a job well done is priceless.