And if I understood the above explanation correctly, you're not actually pumping the water very high at all. It's the difference in the waterlines that matters. The four feet of water above the pump offsets the weight of the four feet of water between the outlet and the waterline. Hence you have very little static head, and unless your filter or tubing is highly restrictive, you may be running the water past the UV so fast that most of the algae cells survive to reproduce. Rigging a bypass so that some of the water goes around the UV would fix this. Be careful not to decrease the flow through the clarifier too much, though. If you can use a stronger bulb or add another UV source, that would be even better.