thank you guys all--both in advance for any insight offered in response to this question and but also for such a helpful resource in general. so we're on a mountainside in the southwestern California desert planning a quick drain/clean/refill. not terribly concerned about water table/popping issues but in recognition of the risk: does the attached image look like a hydrostatic valve? it's dead center deep-end and there's an identical twin in the middle of the spa. has to be, right? I can't tell. thanks so much again and trouble-free pooling to you and yours.