UV lights, yes or no????

I am currently running three UV units, two are 9 watts (part of an internal filter) and the other is 5 watts. I have one that I alternate between the two Koi tanks at work and the other two are at home, one in the goldfish tank full time and the other alternating between my two 20 g. with the tetras and danios.
I DO know they will keep algae at bay...especially in the Koi tanks and will clear the tank in less than five hours if I forget to switch it from one tank to the other. Not so sure they are that effective at home, but I don't have an algae issue with the tanks they are in. I run a 20 watt one in my outside koi pond in the Summer and it is really obvious quite quickly if the unit isn't working for some reason, the algae takes over almost instantly.
My question/concern is are they doing any harm to the fishes??? In my reading I am getting mixed opinions. Some claim they are harmful to metabolism in general, others say no issue at all. Because the lamps are fully contained within the filter housing the fish are not being affected by surface rays from them, which makes it hard for me to believe they are a problem.
I do not expect I am getting any benefit against harmful bacteria or parasites such as Ich because the water is passing thru the light too fast. In fact, I had a pretty severe outbreak of Ich in the tetras about a month ago and it occurred the day AFTER I set up the UV light. Could be coincidental since it was about a week since I had added some tetras from Walmart. Fortunately, I grabbed the right treatment in Walmart before I could do any good research (malachite green and formalin) and used half doses because they are tetras, and only lost one very tiny guy. The rest cleared up almost overnight, whew!
Do any of you in the know readers have an opinion on their use in fish tanks?

Personally I love UV filters, lights or sterilizers whatever you want to call them. As you do I use them outside in my fish ponds and would not have a pond without one. They do a bang up job of keeping the green water at bay. I also use them on my large holding tanks inside. These tanks are mostly inside greenhouses so they get a lot of light and they quickly turn green if the bulb dies.

Someone asked this question a while back and at that time i could not find a reference for it but...

I do remember reading something a number of years ago that said the light had to be at least 18w for it to have any effect on free floating parasites. Even for a 18w the flow had to be relatively slow in order for it to work. They are so cheap now that I go for the biggest on I can get. Bought a great one last summer that is 55w and cost me less then $100 landed and that included a spare bulb. This particular article that I can not find for the life of me actually had recommendations for the max flow rate through particular size UV filters and still control parasites.

I have never heard of any credible information that says the UV filters have a negative effect on the fish.

Steve

A friend has an outside pond and she is always complaining about her green water. Every time I point out my clear water and mention the UV light her response is "those things don't work." However when she saw the Koi in their inside tank with no algae she claimed it is NOT the UV, it is because they eat the algae.
I sent it home with her to use for a few days.
She asked me to help her pick one out for her Wintering inside goldfish.

They might nibble on the green hairy algae in the pond over the summer but Koi have no effect on the free floating algae in the water. Other then produce waste to help it grow that is.

Steve

Yeah, they sure have plenty of fertilizer!

I am in favour of UV lights as a microbial sterilizer. There are several microorganisms that are extremely resilient to many of our water treatments (ie methylene blue, antibiotics and other dyes) so I have been searching and tracking UV sterilizers, in the hopes of purchasing one eventually.

In my opinion, for my application and desired outcome, I would rotate the UV sterilizer based upon need. In the ideal world the sterilizer would be permanent in my quarantine tank, but I am debating upon higher usage vs permanent placement.

The other consideration is the cost for replacement bulbs and I am still tracking the cost on several online suppliers sites.

In summery, determine your need, and your application. For me, it would be as a prophylactic measure against any unseen microogranism and in emergency use, for use in quarantine. As I do not bring in many fish, and very rarely from LFS, I have a very targeted need. UV sterilizers are not for everyone, but big fish owners, koi owners and high value fish keepers will find it a necessary piece of equipment.

I buy the "used" ones on Amazon Warehouse deals, those that say repackaged are brand new, just have damage to the packaging. I get either SunSun or Aquatop, both obviously made by the same company. Unfortunately, you cannot slow down the flow of water thru them so they wouldn't help too much with microbes.
The ones that are 9 to 15 watts are usually around $35 and retail is usually $45 - $50. I am going to try to get one more for the new 55 gallon, but will rotate if I need to since they really only need one day a week for algae control. I am concerned about spreading things from tank to tank, but I guess that isn't a major issue when I use the same Python and nets and stuff like that between tanks anyway.
My pond one was around $75 back before I used Amazon and also had a replacement bulb. However, Krista told me you sometimes have difficulties with Amazon and shipping in Canada.

I love UV Sterilizers for both parasites and algae. I did post a link a while back to a chart of the power required to kill algae and parasites. Can't find it so here's the link again: http://www.drsfostersmith.com/pic/article.cfm?aid=435

I used UVS in all my tanks and never saw any detrimental effects. Although they can be expensive (and the biggest cost is the bulb), what you spend on meds and other treatments can quickly add up to way more than the cost of a UVS. Not only that, think of the hours you spend doing treatments and water changes.

I can't grow green water to save my life, which is a drag as it would be sooooo good for my daphnia. That said, I strongly recommend UV sterilizers as a non-chemical solution to parasites such as ich that have a free moving stage. As others have said, rotate them accroding to need.

I have UV filter units on my pond 36W and 6 - 13W in my larger tanks.
and a couple of 9W ones in grow out tanks.

I have never had a problem with the fish as a result.