Sounds Cool. My only fear in having the drain pick up so close to the water would be sucking air But if you're pushing that much volume through I think it would be hard for them to create the vortex needed.so based on the charts I have found, each 1 1/2 drain pipe from the tank can do a max of 2100 gph gravity fed. So three would be a max of 6300 gph. Which I dont see will happen as with any over flow box or any type of drain cover water flow will be restrict water flow.
Also since I have two Ecotech M2 as the returns their max is 2000 gph for each with 1 inch pipe return.
So I figure my max tank turn over rate is 4000 gph which I don't see a realistic number achieved, since their is head pressure. So I calculated it to be about 3000 gph which gives me a 10 times tank turn over max.
The drains are set right now at 2" below water height. I may change this or change the drain covers. I don't know until I get water in and have the system cycling.
The two sumps can handle a 3" water drop from the main tank which would be 37.4 gallons and they still would have room for additional 10 gallons.
I am allowing a max of 2 inch drop from main which will be aprox 25 gallons.
Once I have water in the tank I will fine turn the whole water flow etc process.