I am borrowing this pic from another thread, but my skimmer looks the same. When we closed last week i just plugged the "back" hole, the one with the gizmo in it in this pic. In my skimmer, the back hole is the only one that blew air. It is also the hole that i attach my vaccum to and from what i can tell, the only hole that even functions.
What is the other one for? I think mine isn't even open if i recall, in other words i think i can see plastic in the bottom of the hole if i recall.