I am late on this subject, but as someone who has worked professionally
with UV Sterilizers (and development) I feel I can add some important
points.
* UV sterilizers do not kill everything, especially many large
protozoan parasites, but when used PROPERLY they are an effective means
of disease prevention (but not the only)
*When UVs do not work it is because of poor contact time or poor UVC
contact with the water. I have gone out on service calls where the
customer has bought a UV from a dishonest or unknowledgeable dealer
that has no chance of working under the conditions presented. Example
(from a real customer): A Lifeguard 8 watt quartz UV connected to a 100
gallon aquarium, run by a 700 gph pump. WRONG! First the Lifeguard 8
wattUV is a poor quality UV with poor light contact with the water;
second, the water flow rate is three times too fast for proper contact
time.
* UV sterilizers also help with the Redox potential which indirectly
helps with fish health via better water quality.
For much more information about how UV Sterilizers work, as knowing how
they work is important in knowing what they can and cannot do., please
read this article:
http://aquariumuvsterilizer.blogspot.com/

Carl