UV can be one of the best investments you make on your tank, or a complete waste of money, depending on how much you researched, and your expectations ahead of time. One such analogy you could make is that it puts sparsity back into a closed system. Normally, fish and all their various diseases and parasites aren’t trapped in a box, and those parasites are not allowed to reproduce freely right back onto the hosts they just left from. Sparsity, in nature, would thin out its survival, evening out the odds.
UV exposure is primarily measured in microwatts over centimeters squared (uw/cm2). That said, UV only kills what passes through it, and different organisms are killed at different exposure times. Algae is easily killed by UV, but a parasite like ich takes a lot more exposure time, something around 336,000uw/cm2 worth of UV exposure to kill, and to get there you need to size a sterilizer that’s both capable of generating such power, as well as maintaining that power at a useful flow rate. Double the flow rate, and you half the exposure time. Slow down the flow rate, and you increase the exposure time. However, you don’t want it so slow as to be ineffective with the size of your tank – and a general rule is you want the full tank volume cycled per hour – so if your tank is 100gal, at minimum, you want 100GPH through 336,000uw/cm2 worth of exposure. Increasing the size of the UV generally means you can increase the flow rate, as long as you’re still hitting that 336,000uw/cm2 exposure time number.
All this is fine and dandy – but now let’s get to the reality of the situation: most aquarium UV’s sold on market are glorified snake oil, with no published exposure time ratings, and power compact bulbs that are inefficient at generating the type of UV-C needed to effectively kill parasites, that they’re just glorified suspended algae scrubbers, and that’s about it. That said – if you have a suspended algae problem, one of these cheap UV’s might seem like a great deal, but some are just flat out unsafely made, and others are so ineffective that simply buying a smaller model from a brand with published-numbers would still be many-hundreds-of-times more powerful than one with a seemingly-higher-rating yet the ineffective power-compact style bulbs.
When we talk about real UV’s, there’s only two brands to really mention, and that’s AquaUV and Pentair (previously Emperor.) Both of these brands use real T5/T6/T8 style bulbs in most of their UVs, not power compacts, and most of the bulbs on these proper UV’s are actually interchangable, as they are not proprietary power compact bulbs, but rather a standard T5 UV bulb. My 25W aquauv has an emperor-branded bulb in it right now, for example. This isn’t a sure-thing, but most of the real UV shares the same stuff to some extent, especially the smaller models. The design they share is pretty much the standard for all the properly-made sterilizers on the market.
As for the benefits, we need to talk about nature versus our tanks, which is a closed system. In nature – ich could be present, but it’s not going to be an infestation, why, simply, volume, dilution, and sparsity, the parasite just can’t reproduce and attach to enough fish to become plague-like in the ocean, however, in a sealed box, you better believe that parasite can reproduce out of control. UV isn’t an ich cure – all UV can do is put your odds a little more even with what they have in the ocean – thinning out the numbers of the “parasites trapped in a box” to the extent that it matches nature (the ocean) a little better, in terms of the parasites sparsity in any given volume of water. And for that matter, it’s not just ich, it’s really any free-swimming living thing that passes through the UV, parasites, algae, bacteria, you name it.
One common myth is that it’ll kill all your bacteria or pods, however, beneficial bacterial lives on rocks, not in your water, thus why you can’t cycle a tank by moving water around. Likewise, pods also mostly live in rocks and crevices and on surfaces, they don’t just free-swim around the tank all day, so while the UV is certainly capable of killing those things if they were to pass through it, in practice, it’s like saying it could kill a fish if it swam through it; that will pretty much never happen. Another common myth is that it can diminish the fishes immune system, essentially turning your fish into the fish equivalent of bubbleboy. Bob Fenner mentioned this myth in one of his books, though he clarified it by mentioning there’s no scientific proof behind it, though he seemed to believe it. I personally do not believe this is true, and would want to see published studies. In my experience UV has a positive, not negative, effective on fish on health, and I think a majority of the people who sized them appropriately would agree with this sentiment.
Yet another benefit, and this is maybe subjective from my own experience – running the output of the UV to my skimmer seems to have increased my skimmer performance, with similar effect to biopellets. I suspect this is due to UV’s oxidizing effect, as well as the effect it can have on redox. And, realistically, I figure anything dead from the UV should now be getting picked up by the skimmer a little better, too. Just something to consider if you do end up running one, although i’ve also heard arguments for running the output directly to the display tank, to avoid “recirculatig” the water through the UV – which is generally undesirable, you want new water through the UV each time, if at all possible, as otherwise you might be diminishing the effective turnover rate.
One final benefit that I’ve seen more infrequently discussed than I would hope – is it’s effect on dinoflagellates and cyanobacteria. This is also possibly related to those previously mentioned effects on oxidization and redox – as this is well known to be a way to defeat cyano and possibly dinos as well, whether it be UV, or perhaps even ozone, a topic for another day.
For freshwater, the UV discussion is slightly different, and is usually focused more on green suspended algae, particularly in ponds, but occasionally in tanks as well. However, freshwater ich is a different game entirely, and often times just raising the temperature to 88f means your ich is cured, meaning other than for green suspended algae issues, there’s sometimes not much point. There’s also just simply not the same livestock investment. To drop $300-400 on a UV for a 40-80gal tank, and often the $500-1000+ for an actually-suitably sized one for a larger 100-200g+ tank, you need to have an investment in livestock that matches, and for most freshwater tanks – you’d spend more on the UV than you would on replacing every single fish in the tank – it’s simply a point of no return. Of course, we care about our fish’s lives, but at some point it’d be like taking out a $10000 insurance policy on a $5 fish, and that’s not very sensible, either.