Passive Heating for "pond"

I have a pond with some turtles that need some mild temperatures this winter. I have several ideas for passive and active heating but none are inspiring me.
My mother-in-law says nothing I do is inspiring. Maybe she's right!

The pond is a little less than 350 gallons and the water temperature is already down to 60* at night. It will go below 50* soon as does my Koi pond temperature. I'm located in north Phoenix, Az.

Any inspiring ideas??

Thanks,
John

How about a solar heater made from black plastic tubing. You can use a dual check valve pump, where each valve allows flow in one direction.
The cold side allows water to come into the tube and the hot side allows water out. A weak spring on each allows the water to expand and then flow back into the pond after it is heated, then the cold water is sucked into the pipe from the other end. Works great for a water bed, could work fine for a pond I would think. The stronger the spring the higher the temp of the exiting water.
You can just lay it next to the pond and then take it away next spring. Also cover the pond at night with plastic.
Floyd
Floyd