Don't know if this is the right forum for this, but can anyone recommend an affordable option for a single or dual stage temp controller? I have a fan on my nano that's lowest setting is a bit too much, and would like a switch that can respond like a float switch but with a temp monitor...

Any suggestions other than what's available at marine depot?

Are you looking for a DIY thermistor - transistor circuit to control the fan?

Doable, and cheap.

Something like this ought to work:

http://www.overclockers.com.au/techstuff/a_thermalfans/

Or more likely this one:

http://www.solorb.com/elect/constemp/index.html

Well, I guess that would work. a thermostat probably would too (maybe one of these is one?), but I was really hoping for a project box already made device that would just gate power to the fan when a high temp was reached. An iceprobe like controller would do if I could plug an ac device into it and it would only be powered above a set point or stage...just like a chiller controller, but maybe the links you provided are just that???

Those circuits are pretty much a chiller controller - this is the DIY forum, so solutions presented here are going to be DIY first. I am not aware of a commercially available product that does what you're looking to do, but if that changes, I'll certainly let you know.

Ranco dual stage controller?
http://www.automatedaquariums.com/ranco.htm

Aqua Logic LCD Electronic Temperature Controller
http://www.marinedepot.com/controllers_aqua_logic-ap.html

if its for a fan, then single stage. I got a industrial controller(athena) of ebay for $30, the programming menu can look complex, but if you just set up the basics, and ignor the rest, its pretty realiable. You also have to get the thermacouple and make sure the controller runs cooling, not just heat.

I use 2 single stage johnson controls A419's on mine. One for cooling via fan and one for heat. They come either you set it up style or already made with plug ends sticking out of them. I am an electrician so you can guess which way I went. I wouldve went with the dual stage to simplify but I had a few left over from a job. The other nice thing about mine is that it is powered by 24V so there is no worry about water and electricity. I had to add a transformer but that was no big deal. If you need help let me know and I can walk you through it.