| typically they will draw a couple of hundred milli-amps. its usually written on the lable of the fan.
you could probably rig the bought thermostat unit up to operate a fan, but to be honest I think its really overkill. Those schematics you found will cost all of about $10 for parts, $20 for a soldering iron, and then a bit of time to solder it all together on a prototype board. If you stuff it up... you'll only have to replace a few cheap components, but if you cabbage the other thermostat, it'll be next to impossible to fix it cheaply, costing you the purchase price plus shipping.
Finally, if you use a thermistor as a sensing element, you can put it on a length of wire, waterproof it and dunk it into the tank. the low voltage means no rsk of frying fish, or yourself. Other thermostats that are not designed for wet environments would probably short out and die.
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Craig | 
