Overflow question

[Socalx45]

Im building 120 gallon and had some build me these overflows. Can some explain to me the vents in the middle and the purpose of them? I can open and close them if i want. Thanks in advance

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[ryeguyy84]

What does the top look like? Is that the only entrance to the overflow?

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[Socalx45]

Heres the top

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[ryeguyy84]

I guess you can choose more or less surface skimming. I believe the water level in your tank when your pump is off in this configuration would be determined by your standpipe height

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[supra400hptt]

I think the theory back in the day was to pull water from a lower portion of the tank, in addition from the surface, for better circulation.