One reason for unclear water is algae blooms, which may appear as pea-green soup or filamentous mats that float to the surface of the water. Algae blooms develop when there is an excessive amount of nutrients (such as phosphate and nitrate) in the presence of sunlight. These nutrients come from tap water, rain runoff, fish waste, and debris.
Establishing a self-sustaining pond ecosystem is the first order of business in controlling algae.
Rocks and gravel provide the perfect substrate for colonizing bacteria. Bacteria eats nutrients that feed algae, which keeps water clearer.
Plants remove nutrients from the water for their own nourishment which helps to deprive those nutrients from unwanted algae.
Broad-leafed floating plants such as lilies shade the pond. Algae thrives in sunlight, so adding lots of lilies to the surface will deprive the algae of its sunlight.
Fish eat insects and, in turn, feed the plants.
Mechanical filtration traps debris. Biological filtration counteracts algae-feeding nutrients.
Recirculating pumps add oxygen to the water and allow gasses to escape. Oxygen feeds the fish and the bacteria which feed the plants and starve the algae.
If you need added algae control, consider adding a UV sterilizer to your system. Some filters come with them built in, others can have a UV sterilizer unit added on. As water passes through the intense UV light which is emitted by the sterilizer it kills micro-organisms (some harmful to fish as well) and ends algae bloom.
There are also some products such as Algae Blocker which tints the water and acts as a type of light filter blocking sunlight as it passes into the water depths, thus depriving the algae of one of its most important needs.
Rapidly growing plants in the summer months require more fertilizer than slower growing plants in cooler seasons. If fertilizer tablets are used, they should be applied directly to the roots of such plants, beneath a heavy layer of mud, to prevent the entry of the undesirable fertilizers into the water itself. Never add products containing phosphates and nitrates directly to the water, as this would certainly encourage green algae growth!