can someone give me advice on setting up a herbie overflow

[Myka]

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If I were you I would drill both drains for 1" bulkheads. They are good for 600 gph, which you wouldn't even want that much in a 72 anyway. I wouldn't drill a return, just run it up and over the edge. Since doing it this way on my 90 I doubt there will be a future that I will drill another return.

Also, your return is not considered to be any sort of considerable flow. In fact, your return isn't even counted towards your turnover when discussing flow in your tank. You should have about 5-6x turnover through the sump for maximum efficiency, and low risk for microbubbles making their way to your display. You use powerheads for flow.