I am doing the math using three IBC media beds 16" deep (water level 14"). By my numbers that gives me a total of 46.66 cu' of GB. Using 7.48 gal/cu' would be 349 gal. If I now figure 40% water by volume, the 3 beds would hold 139 gal when full. To flood and drain the GBs every 15 minutes I must move 556 gallons?
The FT has a 275 gal capacity. It is my understanding the FT water volume should be pumped in/out each hour. At this rate am I looking at pumping out the FT two times per hour?
I want to add on 80 sq' of DWC, which would be about 598 gallons. I don't see a problem there, yet.
My question is, do I need a 600 gal/hr pump?
A 600 gph or bigger pump would be reasonable. (just because the minimum water for for the fish tank is 275 gph doesn't mean you should more far more than that. I'm often known to pump between 2-4 times my fish tank volume.)
Now if you are going to do constant pumping, you should probably do at least a 600 gph pump. If you will do timed flood and drain, you will need a bigger pump since you will need to move the necessary amount of water in some fraction of the hour.
I would go more to be safe, plus you need to figure in volume piping size. It really depends on how fast you want to achieve the desired effects, then you have restrictions, or clogs and the possibility of even expanding for the future. You can always throttle down the excessive water, and it depends on if its constant or timed.
While I agree with the mad german that more flow is good. I will chose different words here, instead of throttling down the excess water. I will suggest that you bypass the excess flow back into the tank with the pump in it. This will provide you with more aeration for the pumping electricity and it will protect you pump from excess strain.
Good point on the pumps TC.
Flows from 600 to 900 gph use 1" tubing (http://watergarden.com/catalog/pond-pump-tubing.html)
From the pump to the discharge will be around 5' rise with maybe 3 elbows.