Ultraviolet light burns cells. Big animals like us let our outermost layer sizzle a bit and interestingly we use some of the short wavelength light to actually make vitamin D. Corals can use some of these wavelengths but will shield themselves against a lot of ultraviolet by using reflecting pigments a lot like we use melanin in our sun tans, as damage control. Ultraviolet light much more easily kills micro organisms including fungal spores, bacteria, micro-algae,and protozoans. If you push water past an ultraviolet bulb slowly enough, the tiny organisms in that water will fry. This is awesome for clearing green floating micro-algae from an aquarium or pond. For aquariums and ponds the ultraviolet sterilizers are basically ultraviolet lights within a quarts sleeve which keeps it dry.  This rests within a tube of water pumped from the tank. The quartz, so long as it's clean, allows ultraviolet light to pass and contact (irradiate) the water flowing by. The prescribed rate is 30 gallons per hour per watt for even the crappiest UV bulbs to kill this algae as well as most bacteria. For this you want the water to turn over every 2 hours and so for a 100 gallon tank you would need a 50 gallon per hour pump and just a 2  watt UV sterilizer. Most small pumps are between 100 and 250 gallons per hour and the smallest UVs are around 5 watts and so you can do teh math and end up killing micro algae and bacteria. Do you really want to kill micro algae and bacteria in a reef tank? If you for a strange reason have an algae or diatom bloom in the water then a cheap power head UV combo from amazon can clear that right up for you but for a well established reef, people often pour micro algae, called phytoplankton, in to enhance the health of animals. The bacteria present in a reef is mostly all synergistic with the community and only opportunistically causing disease when fish are stressed and immuno-compromised.

To kill protozoan parasites such as cryptocaryon (marine ich) it is shown that you need to bring the rate of water flow down to 10 gallons per hour per watt to really cook those swimming stages. In this case you want the water to turn over hourly or more. Lets turn it over 3 times per hour for the case of a 20 gallon quarantine tank.  This means 60 gallons per hour over a 6 watt sterilizer. This hopes that a maximum amount of swimming stages are exposed (flow through) the sterilizer. 

In my experience, even in a bare bottom quarantine tank and with a very high turnover rate you are not going to mitigate cryptocaryon...it's a waste of time. I have not seen any research showing controls vs. effective treatments where the parasite was even slowed down. In high density aquaculture the UV is good for keeping pathogenic bacteria levels down in a way similar to injecting cows with antibiotics before moving them to a feed lot. In our reefs and low density quarantines, this is not the case. If you have a large density populated fish only tank I can see this being very effective at keeping bacterial blooms and possible opportunistic fish infections lower but for a balanced aquarium or nutrient quarantine, I'd like for people who promote their use to explain why.

Here is a link to someone who leans hard to using Sterilisers in every application: The science regarding pond use is valid and well documented however his arguments for benefits to marine reef aquariums are unfounded and in my oppinion biased to selling more UV sterilzers.