So your going to drill it at the top to set your water level. I would go with a 1.5" bulk head to a 1" PVC
. This will give you basically zero restriction. So your GPH
will then be set by your return pump. I would buy a pond pump with like a 350+ GPH
rating and dial it down as needed. Since your head will only be at the most 12-14 inches (you can google a head calculator to see how much it will effect your GPH
) it shouldn't effect your flow much. I could be overlooking something but this basic math seems correct.