making a pond cleaning pump

I'm thinking to build a small centrifugal pump to suck in dirts and algae accumulating at the bottom (concrete) of my pond. Commercially
available centrifugal pumps are too heavy and too expensive. I plan to use an old but working 1/2 HP induction motor directly connecting the pump. Please help me designing the shape of an impeller (about 4" diameter) and where I can get a sealer and suitable bearing.
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