How do I figure GPH for return pump?

[ac]

I've got a new 320 gallon tank that has 2 1" drains in the overflow and 2 3/4" return lines. How do I figure what size return pump I need? I was thinking I would get a slightly oversized one and use valves to reduce the flow, but I don't know where to start. Any help would be appreciated.

 

[broomer5]

AC
320 gallons ?
That's a monster sized tank !!
I agree with Kipass's turnover rate reply - but I don't think your tank is currently able to handle that rate - considering it has just the two 1" overflow drains.
I've got zero direct experience with a 320 gallon tank, but I'm pretty sure that each of your 1" overflows will safely drain about 700 gph ~ maybe more. I've often read where 700 gph for a 1" is a very general rule of thumb that many use in this hobby
It depends a lot on the plumbing below the tank, as well as other things.
If I was in your situation - I would consider having either additional drain holes drilled or increasing the size of the existing ones. If not - you may find yourself wanting additional water circulation - and it will be too late.
Powerheads and other forms or intank circulation are always possible ~ but at this stage ~ you may want to talk to some "big-tank-pros" and see what they think.
Getting advise from those that "run" a 300+ gallon tank would be my choice. Otherwise you may get some confliciting answers.
My largest tank is a 75, and I run two 1 inch external overflows - just for comparision.

 

[krux]

heh i was just about to say the same thing, there is absolutely no way that a 3200 gph pump will drain through 2 one inch holes, unless yer using that pump to go like 15 feet up!
ye either need 4 1 inch holes, or 3 inch and a half holes when you are talking over 2500 gph, more/bigger over 3500 if my memory serves of an old chart i saw somewhere.