There are two considerations. The first is to attach a filter to the end of the suction hose. It consists of a filter element contained in a perforated metal cylinder. But these can get blocked up rapidly especially if there is a lot of floating material in the pond. So it is often necessary to create a 'sump' around the suction. Visualise a large milk crate turned upside down with the suction hose going through the top or side. This will keep the bigger stuff out of the suction area and will cut down on the frequency of cleaning.