I recently completed a home made gravity pond filter, and thought I would post the stages here incase anyone would find it helpful. Materials: 1x 20L "Handi Pail" food grade white plastic bucket 1x PVC 'T' section fitting 1x PVC length 1x PVC length for outlet 1x Irrigation fitting Elbow 19mm Section of tubing/poly pipe Pond pump Filter media River rocks/pebbles Filter media: Irrigation Diverter Hose cut into 2cm lengths Bioballs Filter Wool This design is based off of a commercial gravity filter I previously purchased for another system. Method: Cut a hole in the bucket near the top (10cm) for the outlet, assemble the pvc length (25cm), pvc 'T' section and the small pvc length with a 45 degree cut as the flow end which comes through the hole in the bucket. This serves as the outlet with overflow See Pics 1 and 2. Fill the bucket with your filter medium. I used sections of irrigation pipe, then bioballs, then filter wool weighed down with small river rocks. See Pics 2, 3, 4 and 5. Cut another hole in the bucket ABOVE the overflow (top of the 'T' section on the outlet) and push the elbow joint through the hole, secure in place with a cable tie. This is the water inlet. See Pics 3, 4 and 5. Attach the hose/tubing to the water pump outlet and the filter inlet. See Pic 6. The general principle is water flows from the pump to the inlet of the filter where it flows through the filter media down to the bottom of the bucket where it flows up the outlet section and out of the filter. Key things to make sure of: The filter inlet must sit above the outlet and the overflow, this ensures the lowest point the water can travel out of is the filter outlet, and thus has to travel through the filter media. If the water flow is too great for the filter media the overflow ('T' Section) collects this excess and it passes out the same flow outlet. This system with my pump will circulate the entire pond at least 3 times per hour (taking into account head flow height) in addition to this filter I have many live plants taking up excess nutrients from the water body. Filter cost excluding pump but including everything else including clear poly tubing was well below $50 I estimate around $40 (bioballs were the only material I used that was free) and I am using this on an 700-800L pond. The bucket and outlet system alone was under $15. Any constructive criticism welcome, this is the first I have made but is identical in structure to a commercial model I bought.