Thank you Mark for your very good and quick answer! So in essence, as long as the water moves, even at slow speed, it will not freeze (and yes I have it moving through all pipes/parts of the system). It seems as temperature has been going down in my part of Texas that even though air temp was below freezing, pool temp only decreased by 1F a day or so hence remained quite warm. I am not sure at what air temp, pool water would start freezing but it would surely freeze quicker in the pipes if left immobile so your suggestion of using a tarp is great, not to mention a tarp would protects against UVs that have been degrading plastics fairly quickly already. The light bulb would have to be underneath the tarp to generate enough heat: any idea of the wattage needed?

I have a separate question re bleach in case you know: I wonder at what air temp it would freeze. I have it in the garage currently and add it manually but am thinking about adding a Stenner pump for automatic additions. And I also wonder if muriatic acid freezes. If either freezes and expands, there could be some serious damage ;-(

Thank you!