Question; Are UV Sterilizers Good for Aquariums?
As a generalization, this is a question that really should not be asked anymore based on research of the benefits of Level 1 Sterilization that includes disease prevention & control, fish immunity based on redox balance, and water clarification.

However while there are few reasons not to have a UV Sterilizer on your aquarium, most of the reasons NOT to have one of these devices is based on myths that simply will not go away despite verified controlled use of these devices.
The other reason to NOT have a UV Sterilizer is the plethora of low cost quality “Category C” UV Sterilizers which in truth are only UV Clarifiers made by companies such as AquaTop, Jebo, Green Killing Machine, and others that simply do NOT provide level 1 sterilization; ONLY clarification.
You need to look for a “Category A”, or at the very least a “Category B” UV Sterilizer if you want to see some actual Sterilization results beyond clarification.

Social media and internet users need to beware that most supposed UV Sterilizer Review web sites are nothing more than Amazon Affiliate sites where by publishers write web pages reviewing products sold via Amazon. These writers have NO professional experience whatsoever and in many cases have absolutely no idea what they are talking about!!
An example would be “7 Best UV Sterilizer for Aquarium – Top Picks (2019 Reviews)” [aquariumadviser.com/best-uv-sterilizer-reviews/].
5 of the 7 reviewed are not even true UV Sterilizers, rather Category C UV Clarifiers.
Even more worthy of note is the fact that the AAP Terminator/Advantage Category A UVs are missing. Why is this? the reason is quite simple, the AAP Terminator/Advantage IS NOT ALLOWED TO BE SOLD BY DISCOUNTERS INCLUDING AMAZON!!!

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Back to the subject of this article, I will then look at based on my aquarium service business observations, as well as feedback from others I know in the business as to what are the positives and the negatives. With the negatives I will also look at negatives that are caused by UV Sterilizers of poor dwell time design, poor over all construction, poor quality, and simply poor installation.

UV STERILIZER POSITIVES (based on science and professional use, not here say)

• At flow rates of 20-25 gph, depending upon dwell time inside UV Sterilizer, a UV Sterilizer will kill or render helpless most pathogenic bacteria
• A UV Sterilizer will NOT kill beneficial bacteria since these aerobic bacteria reside in the substrate and filter media in a healthy established aquarium
• A UV Sterilizer will NOT kill copepods in a marine reef aquarium.
  Simple use proves this, as well, the science behind how a UV Sterilizer shows that when correctly plumbed, the UV Sterilizer would not be powerful enough nor would a copepod even get into the UV Sterilizer chamber based on how and where they live and correct pre-filtration prior to water entering the UV.
• A UV Sterilizer WILL improve Redox Balance resulting improved fish immune response
• A UV Sterilizer can help slow or occasionally even stop Oodinium, Ich or other single cell aquarium parasite infestation.
  Level 1 Sterilization will slow many of these parasites, if only by improving fish immune response. Level 2 can even kill some single parasites, although not always completely.
  However I should caution a prospective buyer that even the best UV should not be purchased with the belief this will put an end to future Ich, Velvet, Oodinium or related infestations. Purchase a UV as a tool to aid in control and prevention (keyword: TOOL)
• A UV Sterilizer will clarify an aquarium with cloudy or green water

UV STERILIZER NEGATIVES

• A UV Sterilizer is NOT a cure all to poor water chemistry as per Redox, over crowding often resulting in poor water clarity, poor disease prevention, and other poor aquarium maintenance management
• If not correctly installed AFTER filter inline or in a sump loop, too high a flow rate, poor dwell time, poor UV Sterilization chamber design; you often will have useless piece of aquarium equipment.
  This unfortunately happens way too often.
  I have made countless “house calls” where a customer incorrectly installed the UV Sterilizer or purchased one of the countless junk UVs such as the Jebo or AquaTop and then would tell me the UV did not improve my fish’ health. This resulted in this customer stating ALL UVs are useless, when in fact they made this anecdotal opinion based on incorrect installation or use of poorly designed UVs that are NOT capable of Level 1 Sterilization.
• A UV should not be used while planktonic foods are added to the water column. This is common in many reef aquariums.
  However the UV Sterilizer can still be used and should be placed on a timer so that it is run two hours after the introduction of these foods.
• Many UV Sterilizer have ballasts that do not last (such as the Coralife), or gimmicks such as baffles, twists, wipers, that do little or even lower UV irradiation efficiency.
  Do not waste your money on these UVs, otherwise you may end up with a poor opinion of what a good UV can do for your aquarium.

Here is an excellent video by ReefGrrl that many might find very helpful too:

Got UV? I Do! Let Me Tell You a Little Bit About It

Here is another excellent video showing how the AAP/TMC large aquarium or pond UV is superior over the still excellent Aqua Ultraviolet UV of the same category [due to longer dwell time, price, and parts availability]:

Aqua Pond UV vs TMC AAP Pond UV Clarifier Sterilizer

What is also important to know is lamp/bulb maintenance.
To purchase a UV Sterilizer, including a better model such as the TMC and then to rarely if ever change the lamp will slowly result in a device that does nothing for your aquarium.
With 24/7 use, these lamps will already be at 50% in just 6 months, which is when they should be changed. After a year of continuous use the lamp is mostly useless.

The other issue in our competitive marketplace, is that many if not the majority of online and sellers, especially Amazon and Ebay, sell medium pressure and cold cathode UV lamps so as to make a better profit all the while keep the selling price lower than the wholesale cost of the better high output replacement lamps/bulbs.
To make the mistake of purchasing one of these lamps like so many do based on my home and office visits where the aquarium service client is complaining that their UV no longer works as well or that the fish are getting sick more often, is to purchase a UV lamp with 1/4 to 1/3 the useful UVC output! In my opinion this is self defeating!!!

For more on this subject, I STRONGLY suggest reading this article:
Actual UV-C Emission from a UV Bulb; Aquarium or Pond

Before just taking my word and observations, I suggest reading these resources on this subject:

*Aquarium UV Sterilizer Use
*Redox in Aquariums
*UV Sterilizer Review
UV Bulbs; Problems of Quality?

An amateur/anecdotal article about UV Sterilizers to AVOID that shows a complete lack of real research that unfortunately has been circulated:
blog.marinedepot.com/2017/03/the-truth-about-uv-sterilizers-and-reef-aquariums.html

Here are a few UV Sterilizer that I know from practical use work well and reliably.

*AAP/TMC Vecton, Advantage
*Aqua Ultraviolet
*Emperor Aquatics UV
*Gamma UV

Of the four above, I have had the highest longevity, least gimmicky features, and the highest dwell time with the AAP/TMC Advantage & Vecton UV Sterilizers.

Here are a few Economy UV Sterilizer that I know from practical use work reasonably well; at least level 1 sterilization and reasonable reliability:

*Via Aqua Terminator [discontinued]
*AAP/SunSun CUP Series (not the JUP series which do not provide level 1 sterilization)

UV Sterilizers to Avoid; either due to poor level 1 sterilization and/or poor reliability

*AquaTop
*Jebo
*Most SunSun other than the upgraded AAP CUP Internal UVs
*Coralife Turbo Twist [while a Category B UV, the reliability of the power supplies as tested by professionals make these over priced UVs ones to be avoided]
*Green Killing Machine