I was trying to drain some water out of my pool (High CYA). When I got below the skimmers, I shut both of them off at the pump, but no water would come through. After some reading last night, I'm wondering if this is the problem. Underneath the skimmer basket, the hole closest to the pool is plugged up. This one runs to the main drain right? Does is need to be unplugged? It looks to be a plug with a big square cutout in it.