Making a homemade filter for your fish pond is an inexpensive way to provide a clean habitat for your fish and pond plants. Good filtration will maintain water clarity to keep your fish and pond plants healthy while providing a safe place for beneficial pond organisms to grow. Commercial pond filters are efficient but expensive. You can make your own fish pond filter for less than half the cost in a weekend, using basic materials found in hardware stores and home-improvement centers.
Remove the lid from the box and turn it upside down on a flat surface. Cut a hole in the lid using the utility knife, leaving at least a 2-inch border of plastic intact around the edges of the lid. This is where the water is drawn into the filter. Any box that is at least 6 inches deep and 12 inches long will work, as long as it will fit submerged in the deep end of your pond. When in doubt, choose the largest box that will fit.
Measure the opening you cut into the lid and cut a piece of window screen 1 1/2 inches larger than the opening on all sides. Apply a thick layer of the silicone sealant around the under side of the box around the opening. Lay the window screen flat on top of the sealant and press to secure it, making sure the entire opening is covered. Allow to dry.
Cut a circle out of the window screening that is 2 inches larger in diameter than the PVC pipe. Apply silicone sealant to one end of the PVC pipe, and press the screening to fit over and cover the end. This will keep your pump from sucking in any gravel or batting. Cut a circle the same size as the PVC pipe on the side of the box, at least 2 inches from the bottom of the box. Insert the screen-covered end of the PVC pipe through the hole until 5 inches of pipe is inside and 3 inches remain outside. Secure the pipe by sealing it in place with the silicone sealant. Allow all parts to dry thoroughly.
Place a layer of clean lava rocks or gravel deep enough to cover the PVC pipe in the bottom of the box. Cover the gravel with a layer of plastic pot scrubbers. Fill the remaining space in the box with layers of quilt batting until packed.
Connect the water intake pipe on your pump to the pipe that is remaining outside of the filter box. Use the clamp or hose that came with your pump to secure it. Your pump kit comes with an adapter to fit both 3/4-inch and 1/2-inch PVC pipes. Snap the lid back on the box and submerge it with the pump into your pond. Turn the pump on after all the air bubbles are gone.