The microscopic green algae that swim freely in pond water are too small to be removed by normal filtration. They are the basis of a food chain which would naturally feed all sorts of small animals which, unfortunately, are eaten by fish and damaged by pumps. The algae itself feeds on the phosphate and nitrate that builds up in the system and these chemicals are often present in mains tap water. Using rainwater can help to slow down the rate of algae growth, together with the use of phosphate removers, barley straw products or in the case of wildlife ponds, adding live Daphnia - also known as water fleas. Adding more plants to compete with the algae and add shade will also help and is the foundation of the old 'balanced pond' concept.
For most systems an Ultraviolet Clarifier or UVC is the answer and can be guaranteed to ensure clear water when used correctly. These gadgets work by exposing the microscopic algae cells to lethal UV radiation, causing them to clump together into particles that can be removed by conventional filtration. These units only fail when flow rates are incorrect or maintenance is neglected. An important part of this regular maintenance is changing the UV lamp inside the unit and depending on design, this is necessary every six or twelve months.