wapcaplet wrote:it looks to be a typical layout for relays
There are lots of flavors of relays.
Some would easily be operated from the Arduino's 5-V supply but many cannot.
The relay you selected may be one of those that caused the problem you described.
wapcaplet wrote:How would that be wired?
LadyAda has a tutorial that describes using a (different) FET.
Although, the connections will be the same for the IRLB8721.http://www.ladyada.net/products/rgbledstrip/#usage
Also, she is driving 3 different sets (colors) of LEDs.
You need only one FET to turn on/off your pump-motor.
wapcaplet wrote:I'm guessing Arduino control pin through a resistor to the middle pin of the FET, then 12V +/- on the other two pins.
I think you misunderstand the connections for this FET.
When looking at the printing on the FET: (see the spec sheet - http://www.adafruit.com/datasheets/irlb8721pbf.pdf
Left pin is the Gate. Connect to the Arduino output pin (you chose to use pin-8)
(+5v on the gate causes a low resistance from drain to source, 0v causes high resistance)
Center pin is the Drain. Connect to the (-) of the pump-motor. (+) connection on the motor goes to +12-V.
Right pin is the Source. Connect to both the Arduino ground as well as the 12-V ground.
(or the (-) connection on a battery / power supply)
You have very well written descriptions and good photos of your solar heater.
Nice job, John