selah
I also have a 75 gallon tank with a Mag 9.5, but I chose to use a Lifereef "dual" overflow/prefilter.
Rule of thumb many use is that an overflow with a 1" drainline will pass around 700 gph ( about 11+ gallons per minute )
Two of these then would pass about 1400 gph ( 22+ gallons per minute )
But even going on this can be misleading.
Let's look at three tanks all using the same set up. A single overflow with 1 inch drain, and a Mag 9.5.
A 55 gallon tank
A 75 gallon tank
A 120 gallon tank.
In my opinion, it's the combined area of flow between all of the inner box teeth RELATIVE to the water surface area of the tank that should also be considered.
A 55 gallon is 48" X 13" = 624 square inches of water surface.
623 / 231 = 2.7 gallons of tankwater for every vertical inch of tank.
A 75 gallon is 48" X 18" = 864 square inches of water surface.
864 / 231 = 3.75 gallons of tankwater for every vertical inch of tank.
A 125 gallon is 48" X 24" = 1152 square inches of water surface.
1152 / 231 = 4.98 gallons of tankwater for every vertical inch of tank.
If you had all three of these tanks set up with the exact same sinigle U tube overflow with 1" drain, and all had a Mag 9.5, and you had the tankwater level "just at" the bottom of the inner box teeth ( tankwater just about to overflow into the inner box from the tankwater surface ) and you turned on all three pumps, in all three separate tanks, at the same time ...... what would happen ?
What would happen in a given time period ?
Without doing the math, it's clear that the smaller the tank RELATIVE to the size of the overflow ( available water flow area though the teeth ) the faster the tankwater will rise in the tank, and the more water the overflow would be expected to pass during that period of time .... be it seconds, minutes, hours or whatever.
If you took that same overflow/pump combination and put it on a 10 gallon tank - what would happen ?
The tank would surely overflow.
If you took that same overflow/pump combination and put it on an outdoor above ground swimming pool - what would happen.
The swimming pool surely would not overflow, and you would have just a trickle of water moving over the inner box teeth at first, and finally arriving at some flowrate.
That flowrate is whatever the pump's putting out at the installed head pressure.
If the Mag 9.5 is putting out say 800 gph at 4 feet of head, and your overflow can't handle it - then you have a mismatched overflow/pump.
Your choices as mentioned are:
Do not throttle back with a ball valve, but install a larger tee with a larger bypass after the pump output, and bypass some extra flow back to the sump.
Run two overflows by getting another 1" drain overflow and keep your Mag 9.5
Run your existing single overflow but drop down to a Mag 7
Throttle back on the ball valve until the system works - this I would not recommend for the reasons mburnickas stated regarding increased backpressure, overall pump efficiency, excessive heat and premature pump failure.
I actually run a Mag 9.5 AND a Mag 7 on my 75 gallon tank, and sometimes run both pumps at the same tim. The dual 1" drain overflow handles the flowrate of both pumps.
Note: If your U tube has any entrapped air in the upper most curve of the tube, then all bets are off.
An entrapped air pocket up in the curve of the U tube will dramatically reduce the flow through the U tube.
I doubt you have air entrapped in your set up using the Mag 9.5. It probably blasts water through the tube and forces out any trapped air pockets - but it's worth mentioning.
Finally - how is your drainline plumbed back down to your sump ?
Flex hose ?
Hard piped PVC ?
Turns / elbows / restrictions ?
