Weird thing just happened. Working on installing swimming pool main drains In pool before putting down vermiculite concrete bottom. Draines in, ready to put down pool bottom and we get a downpour. The right drain, equipped with hydrostatic valve and drain tube extending down into gravel, didn't move while left side floated up several inches causing the right drain to unscrew slightly, leaving it slightly angled when water receded. Just wondering if this has happened to anyone else and, if not, just an FYI something that can happen.