Bifferwine
I am a girl
The tube labeled "?" should be left open just like it is. It's not for water to pass through to go to the sump, it's to allow air to escape so the water flows more freely. My skimmer (an Octopus also) has the exact same thing. I don't know if it will still work as designed, since the pipe to the sump is going straight up. It should be at the same level as the skimmer, so the water will take the path of least resistance (and flow through the pipe horizontally). I think if left as is, the water is just as likely to go up the empty pipe (the one marked "?").