My UV was working but my pond has gradually been turning green, why?

Quartz sleeve may have become dirty and requires cleaning. Also make sure you have the right model for your pond size. Ensure your pond filtration is removing fish waste and debris efficiently. Dirt in ponds will affect the efficiency of the evoUV. Check that your pond water volume is pumping through the evoUV every 2 to 3 hours. Only after you have carried out these checks should you contact your Evolution Aqua dealer, who will then be able to advise you further.