With a CYA of 30 and chlorine of 15, your pool is already at shock level. Whther you want to cover or not is up to you--mine is never closed or covered because I don't have trees around to dump leaves in it, and Ihave it fenced so there's no risk of a child or animal falling in and drowning. However, if those things are different for your pool, you might want to consider a cover. In Louisiana, we get veryfew nights when the temps are below freezing, so I just run my pump overnight during those times. That way I can keep the chems right in my pool and start swimming as soon as it warms up again. So you have an option to either just run the pump during freezing weather, or to drain the water from your equipment and actually close your pool. I would say that decision should be made considering how cold it actually gets where you are.