Your choice of a UV sterilizer should be based on on their wattage, the gallons per hour that they are rated at, and whether or not you desire to have an inline UV filter which can be plumbed into the output tube of your canister filter or sump, or a hanging UV filter which can be mounted on the side of your aquarium and driven with an appropriately rated power head.
And yes, they can be used to treat a variety of waterborne pathogens including ich. It seems as though it is more common to see reef tanks equipped with UV sterilizers than fresh water systems.
However, as more freshwater fishkeepers see the logic in using a UV sterilizer to kill bacteria and viruses, as well as to remove algae spores before they can propagate into an algae bloom, the UV sterilizer will continue to grow in popularity for for freshwater aquaria.
For example, I purchased an AquaTop UV IL 10 inline sterilizer for my freshwater 37 gallon aquarium (on sale at Amazon.com - $40 marked down from $60) which is 10 watts and rated for 211 gallons per hour.
This means that this UV sterilizer should be capable of removing the pathogens from aquariums ranging in size from a nano tank up to 100 gallons or so. At least this is based on the UV IL 10's wattage and flow rating.
From what I have read, the bulbs in UV sterilizers vary greatly in quality, and most cheaper units may only operate effectively (specifically if they are plumbed into the output tube of an canister filter), if the water flow is greatly reduced. For instance, the rating of the Aquatop UV IL 10 is 211 gallons per hour, however, in all likelihood, if you were using a canister that could flow 211 gallons per hour, the UV IL 10 would not be able to kill many waterborne pathogens, because the bulb is not powerful to do so with the water flowing so quickly through the canister filter's output tube.
However, if you use a valve (most canister filters now have them plumbed into their intake and output tubes) to adjust the water flow of your canister to say 50 gallons per hour, you would certainly be able to kill off algae spores that would otherwise lead to your tank's water turning green.
As such, using the Aquatop UV IL 10 as this author is, is useful in removing algae spores before they can overrun your tank, thus preventing the green water that has become the bane of many a fishkeeper.
I also use a Vortex Diatom filter a few times a week to polish the water in my aquariums, which removes many harmful forms of bacteria that are simply passed through my canister filters and recirculated back into
So I really don't need the Aquatop UV filter for killing bacteria, which is why I went with this inexpensive model, as opposed to purchasing a more costly and better all around UV system. I use the Aquatop for killing algae spores in order to prevent the algae blooms which cause green water, and leave the removal of bacteria to my Vortex D-1.
However, as for killing different species of bacteria with an inline UV sterilzer, the water flow must be reduced even further than for algae. Simply stated, if you want to flow more water through your filter and still be able to use a UV sterilizer to kill bacteria, you will need a more powerful UV bulb. And not just more powerful, but better quality.
Search the Internet for articles on this topic and you will find that they are plentiful.
In the interest of simplicity, this author has also wondered if any fishkeeper has ever plumbed an inline UV sterilizer into the output tube of a diatom filter like the Vortex D-1 or XL?
Since the Vortex D-1 and XL were essentially manufactured for use as part time - dedicated high speed mechanical filters used to polish the water in an aquarium, and in this author's opinion UV sterlizers should also be used part time (due to their ability to kill both good bacteria as well as bad), the incorporation of an inline UV sterilizer like the Aquatop UV IL 10 (or a better quality unit) with a Vortex diatom filter makes sense.
That is if it does not interfere with the priming of the Vortex filters, which are ideosyncratic, given that you must turn them upside down in order to expel any air that is in the filter before they will operate properly.
This caveat aside, plumbing an inline UV sterilizer into the output tube of a diatom filter offers an ideal way to keep your aquarium water in top condition.
Since I had recently replaced a Fluval 305 that I was using on my Aqueon 37 gallon aquarium with a pair of Eheim 2213's for redundant filtraton (the Eheims were on sale at Big Al's online for $69.99 a piece), I decided to plumb the output tube of the Fluval 305 with the Aquatop UV IL 10 and use the 305 as a dedicated filter/UV sterilizer which I can move from one aquarium to another. And as stated, the same can be done by plumbing an inline UV sterilizer into the output tube of a Vortex D-1 or XL.
The IL 10 is a fairly large unit (15 inches in length) and takes up a fair amount of space when placed directly into an aquarium or when plumbed into a canister filter's output tube. The unit is also incompatible with smaller canister filters that use tubing which is smaller than 5/8" in diameter. I had originally intended to plumb the IL 10 into the output tube of my Eheim 2213, only to find that the 2213's 1/2" tubing was too small to accomodate the 16mm adaptors which were included with the IL 10. Aquatop should include a 1/2 " adaptor with the IL 10 so that it can be used with smaller canister filters.
The company may already do so with its IL 3 and IL 5 watt UV sterilizers.
I enjoy the flexibility of having a canister filter that can be quickly moved from one aquarium to another, given the Fluval's Aquastop feature, and readily disassembled, cleaned and stored for later use. The Fluval 305/Aquatop IL 10 combination accomplishes this.
However, I may eventually plumb the IL 10 into the output tube of the Vortex D-1 to see how well a diatom filter and UV sterilizer work as part of the same system.