Trim the swamp cooler pad so that it fits inside and around all sides of the plant basket to create the filter surface. For a basket that’s 10 inches square and six inches deep, the trimmed pad should measure 46 by 36 inches.
Put the basket in the middle of the trimmed swamp cooler pad and 13 inches away from three of its sides. Fold the pad up and over each of the three sides of the basket, so each is lined with filter padding inside and out. Secure the filter padding temporarily with a clothespin.
Fold the final side of filter padding up and over the fourth basket then stretch it across the bottom. Scatter gravel over the padding, especially around the edges, to hold it in place.
Place the pond pump inside the basket. Fill the plant basket with gravel and completely cover the pump. Place the completed filter at the bottom of the pond. Plug in the pump--unless it’s a solar pump--and turn it on.
Clean the filter by removing it from the pond every month and hosing it down. Don’t remove all the aromatic muck collected in the filter padding, because beneficial bacteria is thriving there--and in the pond water--that help maintain the water quality.