You could put a T and a valve on the return line. Opening the valve would reduce the flow into the tank by returning it back to the sump.
Or if your pump is just a little too much you could add a SCWD Reef Current Wave Maker to reduce some of the flow.
IME I had a 950 GPH
pump with two overflows and still used a valve on the pump output. Never on the input. On the output it is the same as more head height. 600 GPH
may be ok.