Remember that a sump return pump is lifting the water around 3'. Most often this will reduce the output of the pump to 40-50% of its rated GPH
. Your pump will probably deliver around 400-500 GPH
returning back up to the main tank.
You can also pipe in a throttling bypass valve if needed... To do this you need to have a T fitting coming out of the pump, straight through the T will go up to the tank return while the side of the T will have a ball valve plumbed right back into the sump. Some some people plumb the sump bypass to the filter side, some to the pump side, up to you really. If the pump overpowers your drain you just slowly open the bypass valve, sending some of the pumps output right back into the sump.