Simply put, dirt is the main reason fish pond water becomes cloudy. Dirt can get into the pond due to run-off from rainfall, as well as fish activity in the pond's shallows. Certain species of fish, such as carp, feed on the bottom and stir up the soil as they do so. The best way to cure these problems is to establish plants, such as water lilies, around the edges of a pond and to remove any problem fish. Once you have remedied what is causing the cloudy pond water, you need to use a method that cleans and clears it.
Apply aluminium sulphate to the surface of your pond. This should be applied at the rate of 68 to 136 Kilogram per acre. It will bind to the soil or clay particles and cause them to sink to the bottom of the pond.
Add limestone to your pond's water to help maintain the proper pH level. The limestone should be added with the aluminium sulphate at 1 part aluminium to 1/2 part limestone.
Apply calcium sulphate at a rate of 1,000 to 2,000 pounds per surface area acre. Limestone, aluminium sulphate and calcium sulphate are all available at your local farm supply store. For example, if you have a pond sized at 1/2 acre, you would apply 227 Kilogram of calcium sulphate.
Premix all of the chemicals listed above. Spray the chemical mixture across the surface of the pond. The pond should start clearing within a few days. If it does not, increase the amount of chemicals used and reapply.
Clean the water naturally using a bale of barley straw. Use 1 bale of barley for every 1000 gallons of water. Cut a string the length of 1/2 the depth of your pond. Tie one end around the bale and the other end around a large rock. Sink the rock into the centre of your pond. The bale will remain effective as a natural water filter for 6 to 9 months.
Remove any problem fish from your pond. You will notice that some fish are "messier" than others. These fish are the ones that continually root around the edges of the pond. You may need to remove these fish to prevent the water from continually becoming dirty.
Position water lilies around the edges of your pond to help prevent erosion and add oxygen to the water. Remove the lily blooms after they die so that they don't decompose in your pond and add to the dirty water problem.