We have a nice large pond on the farm but the water is very dirty looking and even gets stagnet in hot weather. I have a gasoline powered pump 3 and a half horse power how can i filter the water to make it cleaner ? It has lots of fish in it so I guess it is ok but we sure would love to swim in it. Thanks for any and all ideas. How do they get the sand to stay in a pool filter? Could I build a large box fill it with sand and pump the water thur it?