Flow rate ?

Right I'm sure it is. That's why I said I was using a 1" bulkhead. I don't need any more flow than that.

 

I would go larger than 1" - I have two 75gals and one has a 1.5" bulkhead and one is a 2" with such a large tank I'd go bigger, it'd be easier to do that now then later when you realize you need more flow...

 

I probably didn't mention that this will be a FOLR system not a reef. I'm done with reefs. Turnover at 600gph x2 would be 1200gph wich is more than sufficient.That's more than 5 times the total volume of the tank per hour. Also on your 75g tanks you probably only have one drain and not two so a larger bulkhead would be necessary in that case.

 

Moving water from your tank to your sump doesn't have to be huge amount of water. Really the purpose is to bring water to your skimmer, heater, and maybe through a chiller. Realistically all you need is about the same flow as the pump of the intake of you skimmer.
Another great stand pipe called a Herbie you might want to google it and see what you think.

J

 

Here is a nice baseline chart for such things:
GPM/GPH Flow based on PVC Pipe Size, ie, How much water can flow through Sch 40 Pvc Pipe Size 1/2" 3/4" 1" 1.5" 2" 2.5" 3" 4" 6"