I need to drain quite a lot water from my pool to reduce CYA accumulated by the chlorine tablets, yet it seems that I cannot drain from the wall/bottom outlets only. The attached photo shows the pipes, with the labels added after some experiments by myself. I can turn off one skimmer, but not both. Do I miss anything? If not, do I have to put a plug into each of the skimmer holes? Thanks so much for any help!