Look at the link to the diagram I posted. If pond crud and fish poo are heavier than the water how would it rise from the drain line and be trapped in your settlement chamber? Just doesn't make sense. In a toilet you have water pressure since the water line from the initial flush is higher then the sewer line. In a pond both the settling chamber's water line and pond water's line are at equal elevation and hence there is equalization. Less you stick a pump on the line in between the settlling chamber and the pond with a heavy enough suction force then you can move the crud into the chamber but you would be grinding that crud up same as if you had put a pump directly into the pond.