hello mate welcome to the forum...the only way i can think of is, a gravity fed syphon!1 fill ya main tank to fullest level, mark where the level is. 2 then syphon off water from main tank into your sump to the desired level. 3 with syphon pipe still in place,make a hole the size of a petite pois pea in the pipe. 4 top up main tank back to original level. 5 switch on return pump that sits in the sump...keep a very close eye on this till ya get the flow correct...this will work,BUT AND THIS IS A VERY BIG BUT! if you have a power cut, the sump will fill ok and because of the hole in pipe the syphon will stop! when the power comes back on the return pump will empty the entire contents of the sump into the main tank because the syphon will have stopped which will cause the tank to overfill leading to a flood

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