Ok, I picked up a couple of fans from Northern Tools to use as cooling for my tanks. Here's a link to the fans:
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200308149_200308149
In a perfect world this is what I would like to do (for as cheaply as possible

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I'm handy with electrical, but when it comes to electronics I get lost real quick.
I'd like to have some type of thermostat under my canopy.... set for say... 80 degrees. When it's above 80degF I want the fans to come on and of course when it drops, the fans should go off. I don't think a normal HVAC thermostat would work in this situation b/c the contacts would corrode very fast in a SW application, so finding a thermostat is my first 'issue'.
Second, I'm a little confused when it comes to power sources. I believe the fans are 12vDC 500mA. So for two fans would I need a 12vDC 1A power source?
Third, I'd like to be able to control the speed of the fans. In a perfect world I'd like for the temperature to regulate the speed of the fans, ie; 75deg fans come on slow; 80deg fans go to med; 90 deg fans go to high. But I'd settle for being able to set the fans to half speed and be controlled on/off by the thermostat.
I'd also like to have the ability to add more fans later down the road. Could I oversize my power supply in order to build for addition of more fans down the road? For example 4 500mA fans would need a 2A power supply?
Any ideas or suggestions you guys may have on this would be greatly appreciated.
Thanks for any guidance or suggestions!!