there is no way a 900gph return pump is overflowing a 1 1/2" pvc drain , there has to be something else going on , a 1" drain should be more than enough let alone a 1 1/2" drain , you shouldnt have to turn down your return pump.
im running 2400gph after 3ft of head through a 1" pvc and i have the pipe restricted about half way with the gatevalve.
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