Overflow for 75 gallon tank.

[Bigfishnut]

Sorry to go off topic but for steele pipes wouldn't there still be a difference between column 1 and 2 given 1 is gravity and the other pressured?

Back to PVC for OP, so if a Herbie drain was used, would he be looking at column 2 most likely?

I think he meant you would use the chart the same for 1 and 2 but 3 would be higher.

 

[Fish Tank Travis]

I think he meant you would use the chart the same for 1 and 2 but 3 would be higher.

Yep

For steel versus pvc the only difference between column 1 and 2 would be the friction of the pipe wall, since they are both the same size. However, since steel can hold a great deal more pressure than pvc it will be able to flow a lot more under max pressure, which is column 3. More pressure equals more flow.

For the OP, we will be using column 1. Even a herbie style overflow is just gravity fed. Only way to look at column 2 or 3 is to add a pump or some sort of pressurized flow. This is why sump setups always have smaller return lines than overflow lines, because they are pressurized with a pump that allows them to flow more for the size when compared to gravity lines. It also helps to keep the return flow from being able to overwhelm the overflows.