With money being tight I couldn't afford a fancy pond filter, instead I made my own.
I used treated pine sleepers to make a box. I went 3 sleepers high I lined the box with old carpet and some insulation foil that I was given. The carpet etc are only there to protect the pond liner I used from being pireced by a stray splinter. I then put off cuts of pond liner over the inside of the main liner to protect if from the river rock I used. I used 50mm pvc pipe as an inlet at the back of the filter. This inlet pipe went down to the bottom and then a 90 deg elbow to bringit back along the bottom towards the front of the filter.
Along the inlet pipe at the bottom I have 5 x 50mm outlets running either side of the filter. Then at the front of the filter on either side I have 2 x 50mm tank fittings as outlets which are connected to 50mm pipe which join at the bottom to a single pipe leading back the the waterfall in the pond. The whole thing was filled with small river rock ( 1 cm3 ).
I tried to clean the gravel as it went in but a almost impossible task. I run two 2000lph pumps through it when I am running the pond and skimmer pumps together. 99% of the time I just run the pond pump through it ( 24 /7 ). Well when I first turned on the pumps the water looked like really thick pea soup. Just over one day later I could count pebbles on the bottom at the deepest point (600 mm ).
That was 4 years and a bit more ago. Never cleaned the filter and the water still runs crystal clear with 3 turtles numerous yabbies and fish The pics show when I built it and the only thing I changed was the inlet which only had a "T" piece then. Now has a pipe running back to the front and 5 outlets. I planted out the top so it now just looks like a raise garden bed with a couple of pipes comming out.