How to Size UV Lights for Ponds

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Ponds are in a constant need of maintenance to keep algae from taking over. Algae grow best on warm, sunny, open-water ponds. Installing an ultraviolet (UV) light can help dispel some pond algae problems. UV light breaks down the microscopic cell walls of algae, which forces it to clump together in bigger groupings. Once algae starts gathering in clumps, a good filter can sift it out more effectively. The light facilitates that process. However, you definitely want to choose a UV light with a high enough wattage for your pond size.

Skill level:
Moderately Easy


  1. 1

    Record the daily amount of sun exposure your pond receives. Detail if it is full sun hitting the water or if there are pond plants or other features partially shielding the water from the sun.

  2. 2

    Estimate the fish content of the pond. List approximately how many of each kind of fish live in the pond. Note which kinds of fish you placed in the pond and how long those fish have been in the system. The longer the fish have been present and the larger they are, the more nutrients for algae they produce with their waste.

  3. 3

    Look at the gallon capacity of the UV light and compare that to the size of your pond. Low wattage and low flow capacity will not accomplish what you hope for, even in a small pond. If your pond size falls between two models, choose the larger model.

  4. 4

    Choose a UV light wattage that estimates up, rather than down. If your pond is highly sunlit, full of fish and generally accumulates a large weekly quantity of algae, choose a higher wattage UV light to accomplish the algae cleansing. The more factors affecting the algae-producing nutrients of a pond, the higher your UV clarifying light needs to be for effective cleansing.

  5. 5

    Clean or rinse the filter each week to keep the algae removal at its highest capacity. Since it is being filtered in larger chunks, the filter will clog more quickly and will require more cleanings initially to work most effectively with the UV light.

Tips and warnings

  • Look at several brands to compare the UV light wattage and gallons of flow before purchasing a UV light.
  • Be careful that water flows at a consistent speed through the UV clarifying unit as a flow too fast or too slow can hinder the UV performance.
  • UV lights are not substitutes for pond filters to clean your pond effectively.

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