copied from the web
What exactly is UV sterilization? It's actually a very simple process for removing (referred to as filtering) unwanted free floating microscopic water borne bacteria, parasitic, fungal, viral, algae, and other unfriendly pathogens out of aquarium water by exposing it to high intensity ultra-violet (UV) light. UV light has the ability to effect the function of living cells by altering the structure of the cells nuclear material, or DNA. The end result is the organisms die off, eradicating your aquarium water of these unwanted nuisances.
There are three types of UV sterilizers; tray type, tube type-wet bulb (non-quartz lined), and tube type-dry bulb (quartz lined). Suggested wattages are 4-8 watts for 20-40 gallon aquariums, 20-25 watts for 50-100 gallons, and 40 watts for larger aquariums. Using this method of treating water is quite controversial amongst aquarists. Some consider it to be a waste of money, not all that effective, or suitable for fish-only, but unnatural for reef tanks, while others feel it does have its benefits for either type of system.
The general consensus is that an aquarist that has a well maintained and uncrowded aquarium, as well as follows good quarantine procedures when introducing new specimens to their system, a UV light filter is not necessary. For those that may be deciding on whether a UV light filter should be added or not, here are some other points to consider.
* Most effective when run 24/7.
* Most effective if the water is clear.
* Most effective if bulb is new, or replaced regularly (at least every 6-8 months).
* Most effective if the UV light penetrates less than one inch of water.
* Effectiveness can be hindered if the water passes to fast past the bulb. Most effective if the exposure time of the water to the UV light is longer than one second.
* The effectiveness of UV light can be hindered if there is light blockage, i.e. a salt encrusted bulb.
* It can help to prevent future water borne pathogen reoccurrences, once the initial problem as been completely eradicated from the aquarium.
* UV light not only kills unwanted organisms, but beneficial ones as well.
* Because it can destroy beneficial microscopic organisms that some reef tank inhabitants may depend on as a food source, UV light should not be run during feeding time.
* It only destroys organisms that are free floating IN the water as it passed by the UV light, i.e. it will NOT get rid of an ich infestation that is already ON fish, or cure a bacterial disease a fish may have.
* Should never be run when treating with any drugs or medications.
* UV can also alter the structure of some dissolved chemical compounds.
* UV light can be damaging to the human eye, so DO NOT look into the bulb.
* Always unplug the unit when working on it to prevent possible shock if it breaks or gets wet.
