If a pond stays green despite a UV unit, troubleshoot the water and the installed circuit before buying a larger lamp. The main UV guide explains selection; this checklist helps find why an existing system is not delivering the expected result.
1. Identify What You Are Seeing
Suspended green algae can respond to UV clarification. Brown suspended sediment, tannin-colored water, and string algae attached to walls are different problems. View a sample in a clear container and compare the color and particles with what remains attached to surfaces.
2. Check Power and Lamp History
Confirm the unit operates according to its instructions. Record the lamp's age, operating hours if known, and replacement part number. A visible glow is not proof that the lamp still supplies its rated treatment output.
3. Inspect the Sleeve Safely
Deposits reduce light transmission. Disconnect power, isolate the unit as directed, and follow the manufacturer for opening, cleaning, and checking seals. Never inspect an exposed operating UV-C lamp. Check for leaks after service.
4. Verify the Water Route
Trace the actual plumbing, including bypasses. How much flow enters the unit, and is it within the exact model's clarification rating? Too much flow can reduce exposure; a very small treated branch may not address the pond load adequately. Match both exposure and total treatment circulation.
5. Check Filtration and Debris
Remove accumulated solids, inspect mechanical filters, and check overall circulation. A UV unit cannot compensate for every waste problem. Continue ammonia and nitrite tests while the water is green and after it clears.
6. Change One Relevant Variable
Record a starting photograph, the maintenance performed, branch flow, and subsequent appearance. Allow the model's stated operating period under suitable conditions before judging a change. Do not repeatedly add algaecides while ignoring oxygen and the underlying cause.
Clearer water makes observation easier, but it does not prove the pond is free of disease. Reference the manufacturer's UV guidance and your unit's manual. For general waste and water concerns, see water quality.