The Natural Approach - Keeping a healthy balance of pond life will result in relatively clear water. Scavengers will consume filamentous algae and decaying plant material. Heterotrophic bacteria and other microscopic life will help to break down other organic debris. Excessive water changes, drastic cleaning or harsh chemicals will disrupt this balance.
Re-circulation - Allowing water to splash back into the pool will oxygenate the water for healthier conditions (i.e. Waterfall and, or Fountain). To avoid plugging up the pump, some simple filter should be used at the pump inlet. This will assist in removing material which could cloud the water. Caution: water-lilies like still water so minimize or isolate the fountain or fall.
Bio-filter units - These units generally
have a "mechanical" element, a porous pad that strains out
some of the particles and another (generally plastic) element that provides a surface area for naturally occurring bacteria. The external bio-filters are larger, located outside the pool for easier cleaning, and require the addition of a suitable pump and pre-filter. Smaller in the pond units may be an economical choice for the beginner or for the small pond. Their location and their limited filtration area make maintenance more of a chore than larger external filters.
Wetland Filtration - A small separate pool is used as the wetland area, stocked with a mix of emergent aquatic plants planted in gravel filled baskets. The gravel provides surface area for the nitrifying bacteria. The roots of these plants act as baffles, causing particulate matter to precipitate to the bottom of the pool, while they also remove nutrients from the water. Generally, the water is spilled or run back to the main pool, thereby further oxygenating it. A pump and gravel pre-filter is required.
Simple (or Mechanical) Filter - Generally
a foam or sponge-like pad through which pond water is
pushed or pulled . The pad physically strains out solid particles as the water passes through. Vortex filters and settlement chambers are designed to utilize gravity in place of a pad. Proper selection of a filter pump improves Mechanical effectiveness.
Ultra Violet Clarifier (U.V.C.) - Unlike a high powered U.V. Sterilizer, the U.V. dosage delivered causes planktonic algea to clump together into larger particles without harming other beneficial micro-organisms. Higher powered U.V. Sterilizers have long been used in the aquarium industry to kill any pathogens passing through - its throwing the baby out with the water!!
Whichever option(s) you choose it is important to tailor it to the size of your pool and your ecological system and to consider the esthetics of its location as well as access for for periodic cleaning.