Never return flow back to the sump. This actually makes the pump work harder, create more heat and draw more power since it has to pump all that water you are wasting plus what goes to the display. Very very old technology not used much anymore now that we can monitor power and flows easily.
Instead, install a ball or gate valve inline on the discharge side of the pump and throttle the flow to match your needs. Since centrifugal pumps only draw the power needed for the work at hand and you are requiring less flow, it is drawing less power and creating less heat and does not hurt the pump in any way, in fact on many pumps it puts them in their "sweet spot" and they are more efficient. I know it doesn't make sense but it has to do with hydraulics and centrifugal force and is spelled out in many electrical and hydraulic theories and laws.
A very often used rule of thumb is to provide 3 to 5x your display volume after head loss with your return pump and I lean towards the 5x side. So a return rate of 450-500 GPH at probably 5 or 6 feet of headloss is about right. The Mag 9.5 should fit that but I personally hate Mag pumps due to their higher than normal power consumption, high heat generation and are quite loud. There are better, more efficient and quieter pumps such as an Eheim 1262, Ocean Runner 3500, Octopus 3000 or WaterBlaster HY-3000 that would fit the bill and still have a little extra flow if needed.
Never restrict the overflow! This is a disaster waiting to happen. All it takes is a snail to lodge in there and its all over. Wide open always. You can install a valve for cleaning and maintenance but its really not needed since the standpipe and overflow will be empty when the power is off. The overflow will always balance itself out with the retrun pump if it is sized properly which yours is at 1.25". The retrun pump determines the overflow and return rate all by itself. The flow is listed on every pumps curve or data sheet. Look at the pump you want then scroll down the sheet to the headloss you calculate (often around 5-6 feet including fittings, height from water surface to water surface between the sump and display and line losses) then across to see what flow you can expect.
Turnover usually includes the return pump/overflow GPH plus any powerheads or closed loops you have too so will be much more than the 3-5x the return provides by itself. A turnover of 15-30 or even 40-50x total is very common when you include a couple of 1000+ GPH powerheads like Evolutions, Tunze, WP or MP etc.
Fred
