Thanks for your replies.

Clarifier after the filter... thats interesting. I checked the manual and doesn't even say where to put it! I thought I heard the guy at the garden centre say where to put it, but I could be mistaken. I remember now, I think he said "the UV filter will kill the algae then the bacteria in the filter will clean up the dead algae" or something like that so I assumed UV first, filter second.

I'll have to call them up and ask them.

The manual does recommend a Tetra pump to use with this filter, perhaps my pump is too fast or something (I read in another post someone saying the flow could be too fast for the clarifier).