I have 2 openings in my only skimmer. One is open & working fine.
The other opening is really a dummy, it's solid. When I winterize I do close it with a winter screw-plug (just to have less clean up tasks at the spring opening & just as well because it could fill & freeze I suppose).
Is that necessary to do in the winter? , and
Is it best to leave it plugged in summer or not or makes no difference?