Even after twenty plus years, I still see these units being touted as a miracle cure all for parasites, disease and algae problems / preventions. They are nothing more than a symptom cure at best, and a false sense of security at worst. I have had extensive experience with these units ranging from single tank units up to industrial room sized units used within entire warehousing and shop tank systems. All of which failed miserably at their intended and touted benefits.
I believe the debate over the use of these units stem from a simple misconception that they, and they alone will perform a cure of what ever you are being told or have been told they will cure. They are not a cure, at best, they are an aide in an overall strategy in defeating water borne problems. Other measures must be taken to actualy affect a cure. You will be disappointed if you think that by just putting a UV Unit on your tank, all algae and parasite problems will disappear.
I also believe that while they may not be of great use on an established tank, they can be of use in a quarantine / hospital tank set up. Given that most newly acquired fish are weakened by stress, any reduction of a pathogen, how ever slight, by a UV unit is still a benefit.
My following opinions are meant to emphasise the importance of getting the word "cure" out of your head when thinking of UV Units, they are not a cure, only an aide to slow the symptoms of a problem, and do not address the cause of the problem, find that cause, fix it, and you have a cure.
Parasites - Given that any parasite that is introduced into our tanks is being put in with a captive audience, and usually always already attached to a fish as new additions that have not been quarantined, such an enclosed space allows any parasite or newly "hatched" parasites ample time to find a fish long before it happens to run into a pump intake leading to a UV unit. With the sheer numbers of possible parasites being produced or introduced the odds of a UV unit being able to kill each and every single one of them before being able to get near a fish is so far fetched that I find it amazing that people still believe it is possible to do so. There is only one single method that will ensure such parasites remain out of your aquarium, and that is to set up and follow a strict quarantine/treatment procedure for all new livestock. If they were so great at parasite control, then why are we still bringing parasites home to our tanks since most shops use them within their store's system, and the shops that do not use them, have stopped doing so because they learned an expensive lesson the hard way. They are useless.
Disease - This same thinking falls right in line with parasites. With the vast amounts of bacteria and virus' that are or could be present within our aquariums, just how good do you think these units are? It would take the total sterilization of our tank's water to ensure that a given bacteria or virus could never get on or in one of our fish or corals. Think of the odds involved. Again, it amazes me the thinking involved as to how a UV unit can actually perform that feat. Quarantine procedures and healthy living conditions are the keys to prevention.
Algae - Yes, a UV unit will clear up green water due to free floating micro algae'. But think about that one for a minute, it does so by killing the algae along with what ever else passes through it. What happens to all that dead matter now? It rots right back into the nutrients / organics that fueled the algae outbreak in the first place. A cure? I dont think so, using a UV unit for this purpose is just a quick fix for a deeper problem that has not been dealt with, namely, nutrients. Get some proper nutrient controls in place and you can put that UV unit up on Ebay for people with outdoor fish ponds. For algae control, a UV unit is just a vicious circle. For cleaning up an algae outbreak, you will get much better results with a Diatom Filter, its use actually removes the algae from the water along with what ever nutrients that algae has within it. But as they should be, these diatom filters are used on a limited basis to clean up after a tank nutrient disaster or keeping things cleaned up while other nutrient controls are being allowed to take effect.
Reef concept - Another area of concern is what I would consider one of the most important reasons for not using a UV unit, in that they kill everything that passes through them! In a reef aquarium, what does most, if not all of the life we put into the tank eat? Alot of micro life, which includes phytoplankton (algae!) and zooplankton, the same reason that I do not use any type of mechanical filtration other than a skimmer, why would I want to trap and or kill all the free food that my tank produces? Sterilizing a reef tank's water goes totally against the whole concept of keeping a "reef" as a bio system. On top of losing all that free food, there are a great many creatures that use our tank's water as a way to expand their populations into new tank territory, such as sponges, tunicates and a huge host of other important and just plain "cool" creatures. In my opinion, if you use a UV unit, you might as well just pour a cup of bleach into the tank each week.
In Short : A reef tank - No, I would never use one.
A fish only tank - Yes, I see no harm in having one on such a system.
A quarantine tank - Yes, every little bit helps.