See:
http
/www.americanaquariumproducts.com/AquariumUVSterilization.html
http
/www.liveaquaria.com/general/general.cfm?general_pagesid=21
Both of these websites are also attempting to get one to buy a UV sterilizer. That's doesn't mean that their information is wrong, but it definitely is unbiased.
Regarding the larger question, this has been debated very recently.
Many of the facts remain the same:
1) A UV cannot treat the entire tank volume at once. This is because only a small volume of tank water that is treated at any one time.
The result of this is that UV cannot completely remove 100% of a microbe infecting a tank.
The concentrations of microbes in a tank is described by the following equation: dc/dt = B - D. dc/dt = change in concentration of microbe. B is birth rate. D is death rate. In words the equation is:
The change in concentration of the microbe is equal to the rate of births minus the rate of deaths. If the rate of deaths is greater than the rate of births, the concentration does down, etc.
A UV can significantly increase D, the rate of deaths. But, so long as there is a finite B, birth rate, then the concentration will never be zero. Indicating that a UV alone cannot ever completely cure a tank of the infection.
As such, I think a UV can be a useful tool to initially treat an infection and decimate the population. Then, as a second step, remove the UV, and then use medication that can treat the entire tank and completely kill off the microbe.
In fact, a UV can give you a false sense of security of having a disease beaten, when it may just be residing at asymptomatic levels because the population is small but non-zero. Then, if the UV is turned off (because of a power cut) or you just stop using it, you may really wonder where this disease came from. It may very well have been in the tank the whole time, and only had a chance to grow when the UV stopped killing off 95% of it.
2) There is a significant cost in replacement bulbs and electricity. And, for the bulbs to have intensity enough to kill microbes, they have to the replaced before they burn out. Usually they lose microbe-killing effectiveness in a matter of months.
3) With good use of a quarantine tank, the amount of infectious microbes being introduced to the tank should be very, very small.
This is for two reasons. A) Almost all the microbes that attack fish, have to have fish in their life cycle to survive. That is, unless a microbe swimming around in the water finds a fish to parasatize off of, it will die. A good example is ich. If ich in its free swimming stage does not find a fish to latch onto, the ich microbe will die.
There are a very, very few fish diseases that can lay dormant. In the marine world, crypto is one. But, it is very, very few.
So, know that there aren't fish swimming about in the water at your water company and there aren't fish swimming about in your underground water well. Because there aren't fish there, your water source has what really amounts to a zero percent chance of containing fish-attacking microbes. The conclusion is then that most microbes that attack our fish are introduced.
So, this leads to the second reason B) if you quarantine any new additions for a long enough period of time, any diseases they could have would show themselves and you can treat the q-tank and make sure that the diseases don't make it into your main tank. Long enough, in my opinion, is on the order of 6 weeks. And, it also means that if you buy live plants and the water the plants were stored in is shared with the fish at your LFS, you need to quarantine plants too. Plants could be carrying fish diseases, but again, if they sit in the q-tank without fish, the fish-attacking microbes will die. The same with other non-fish creatures like snails or shrimp. They could be carries for something like ich, even though they cannot get ich themselves.
Obviously, there is a cost in running a q-tank. But, you don't need to run it continuously, and a q-tank can be ran very cheaply. It really only needs to be a large plastic tub, a heater, and a simple small filter, like a sponge filter. Just set up and tear down as you bring in new fish.
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Because there are other effective means of preventing infections from getting into your tank, I just don't see a need to run a UV full time. I think that they can be a very useful tool to have if an infection breaks out. Hook one up and run it for 2 or 3 days to really decimate the population of infective microbes and knock their population levels down. But, then turn it back off and use effective medication to finish off the job. A UV can be the 1 in this 1-2 punch. The medication will be more effective because there won't be as many microbes that will need to be killed. You may also not need to medicate for as long.
All that said, if you use a q-tank correctly, you won't ever need the 1 in the 1-2 punch, because you won't have infections in the first place. Q tanks can be very effective. As a simple example, I've quarantines for many, many years, and I've never had a disease in my main tanks. I've had ich infect fish in a quarantine tank before, but never has ich in a main tank. The really great thing is that because I only had to treat the q-tank, I needed 1/10th the medicine to cure the fish. I.e. instead of treating 50 gallons, I only had to treat 5.
Lastly, UV sterilizers for fishtanks are relatively recent invention. Think about how long people have been keeping fish and never needed them before. There isn't any compelling reason that they are suddenly needed now. All of the above information tells me that they are a luxury item. Yeah, they can do some good some of the time. But, they certainly aren't needed 100% of the time, and running them 100% of the time doesn't seem like a good use of money to me.
