I'm a new pool owner and just picked up the MX8 side suction to help keep my pool clean. After installing it I noticed it was running slowly. The manual says to close the main drain. My question is how do I do this? I only see one knob on my system and I have no idea what it does. Here is a picture. Any idea what the knob is for? Is that the main drain valve? I only have one skimmer.