What is the difference between a UV steriliser and a UV clarifier?

The very simple answer to this is gapping. Sterilisers have a chamber that brings water and the organisms within it into very close contact with the lamp – this gives a much higher dose of ultra-violet radiation capable of killing parasites that would survive the lower doses used to kill algae cells in a clarifier.

Because of this difference in exposure, a steriliser will effectively function as a clarifier but may be more susceptible to fouling when large particles of waste are pumped through the unit. This is less of an issue for a clarifier that has a larger contact chamber and less restriction on solids passing through it. This is why most units designed for pond use are clarifiers and better able to cope with a dirtier environment.