Ben Helm says...

Installing a suitably-sized UV unit is a guaranteed way of creating clear water in a pond. It works by causing the single-celled microscopic algae cells to clump together into larger particles that can then be removed by mechanical filtration. Anyone who has retro-fitted a UV unit to a pond will have noticed the sudden increase in debris (flocculated algae) that a UV unit can create. Algae (green water) blooms in a pond as a result of there being a high nutrient (nitrate and phosphate) content in the water and therefore addresses the symptom rather than the cause. Consequently, when the UV unit has created a crystal-clear pond that is still high in nutrients, in moves blanketweed to capitalise on these growing conditions. A UV unit will not eradicate blanketweed as it does not pass under the radiation of the lamp, but remains and grows within the pond.