UVC

From Koipedia

Ultra Violet Clarifier, sometimes also called an Ultra Violet Sterilizer, are used to kill floating single cell algae which significanly improves the clarity of the water. UVC will also kill off some parasites.

The only difference between a Clarifier and a Sterilizer is how fast the water is passed through it. The less water that is passed through, the more time it will spend in the Ultra Violet light, and the more parasites will be killed off. Single cell algae is not that difficult to kill so more water can be passed through if that is the only goal of the UVC.

Many ponds are fitted with UVC's these days to remove problems with 'green water'.