Having seen your pics and how your system is set up I can't see how you could have a stockman standpipe. They are designed for a weir type overflow box, not a drilled overflow like yours. The only things I can suggest is try turning the intake on an angle or if possible reduce the return pumbs flow rate so its not overloading the durso. What concerns me with your water level is how much water will syphon down to your sump before levelling out? Have you turned it off to see? Might flood your sump. If it doesn't flood your sump then you're safe.
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I come from the land down under.....