I'm still getting things squared away. Here is where I got the snap switches.
Fan and Limit Controls
Snap Disc Fan Controls , Fan Controls , White Rodgers Fan Controls - PexSupply.com
Since I'm using Firestone 45 mil EPDM Pond Liner, I need to keep the temp limited to 140F to get long life from the liner.
Need to base the temp control on what your water storage container can tolerate.
I plan to use a bi-metal snap switch to set off an alarm (a 12v light) if the temp gets to high.
Hope to go this rout, later on with a differential thermostat controller
12 Volt Differential Thermostat Controller