I have two skimmers and a drain on the deep end of the bottom of the pool. I have two holes in my skimmers and I am finding conflicting info online on whether I need to open both. The hole closest to the pool has a square indent in the plug so I would have to find a tool to unscrew it, if I should open it. I already unplugged the other one. How do you determine if you need to open the 2nd plug?