hi we use a 50/50 mix of distilled water and anti freeze all year round with no problems
it needs to be distilled water not de-ionised or filtered and then you wont get any growth in the tube.
the one thing you may get like we do is if the water goes below 5 degrees the CW 5000 will go into alarm state
al we do to correct that is to switch off and back on till it alarms again and keep doing that till the pump itself gets warm and heats the water slightly then you are good to go for the day
Dave