Just because it is a local fish, it may still die in your system. I had some red ear sunfish die in my system when it got too cold. The waters around here are heated by the power companies dumping waste water into the rivers and lakes. Also, very large bodies of water take A LONG TIME to cool down with only a fractional temperature gradient change like you describe or that we have here in FL. In other words, if it spikes down to 25 degrees, the rivers might only drop from 50 to 48 but your system will drop down towards 25 since it is so small.