The purpose of this article is to provide basic answers to the most common questions asked about the use of a UV Sterilizer in Aquariums

Much of this information is courtesy of:

"UV Sterilizer Use & Information, A to Z"

This article linked to above is probably the most in depth and accurate article on the subject of aquarium UV sterilizer use I have read anywhere.

The author of the above article is one of the leading authorities on this subject with a considerable amount of hands on experience using and even designing true UV Sterilizers as well as UV Clarifiers which are commonly marketed as UV Sterilizers.

Readers will also note that it is often updated too, making for the most current information. 

A UV Sterilizer is a germicidal filtration device that uses UV-C Sterilization to filter the water resulting in clarification, lower disease pathogens, and removal of oxidizers in the water column resulting in improved Redox in your aquarium or pond. 

A correctly installed and well designed TRUE level one or two capable UV Sterilizer can kill most suspended bacteria and many viruses in the water column, while not affecting bacterial colonies in filter media and substrate.

As well a UV Sterilizer/Clarifier can clear green/cloudy water in aquariums or ponds.

See this link for one source: UV Sterilizer/Clarifiers 

A QUALITY UV-C Steriliser also improves the Redox Balance in an aquarium or pond by lowering oxidizers and thus oxidative stress in the water column which in turn improves the fish’ ability to fight disease.

A UV sterilizer aids in the control of parasites such as Ich or Cryptocaryon by improving immune function and disease resistance (via Redox). At very low flows (under 8-12 gph per watt) a UV Sterilizer can more directly aid in parasite control of parasites in the water column.

Unfortunately many UV Sterilizers do not adequately improve Redox balance by lowering oxidizers in the water column that can be dangerous to fish over time.

This is due to insufficient 254nm UVC exposure inside the unit from poorly designed units along with incorrect flow time and use of poor UVC generating medium pressure UV bulbs that most lower cost UVs commonly sold by discounters now use so as to keep the price down. This unfortunately common with many of the low cost models made in China and sold by discounters which in turn then give true UV Sterilizers a bad name.

The picture above provides a visual of how a TRUE UV-C Sterilizer breaks down oxidizers, which are negatively charged ions (anions) in the water column. This in turn reduces oxidative stress on fish.

Also see this article for advanced aquarium keepers:

Aquarium Redox Balance 

A pump pushes the water through the UV Sterilizer at a specific flow rate, where the water will come in contact with the UV-C producing light either directly or indirectly via a quartz sleeve.

The UV Light then sterilizes the water by altering the DNA of organic matter as well as "clumping algae" in the water, and it is returned to your tank or pond. 

Your flow rate is dependent on the level of sterilization you desire from your tank. The slower the water moves across a UV bulb that is intended primarily for UVC production; the more time UV-C is in contact with the water. The slower the rate, the more sterilization you can achieve.

This goes hand in hand with a turnover rate. 

Your turnover rate is the rate at which all your aquarium water is filtered in an hour. For UV Sterilization, you want a turnover rate of at least 1 1/2 times per hour. So if you have a 10 gallon tank, you want a turnover rate of at least 15 GPH. This will achieve level 1 sterilization when combined with the correct flow rate assuming other criteria are met as well.

See this resource for more: UV Sterilization; Turnover Rate 

The flow rate is the most important factor and along with turnover are easy factors to measure.

Nonetheless, while flow rate is probably the most important, probably the next most important factor is not turnover, rather it is water turbidity which includes pre-filtration.

With high water turbidity due to poor or no pre-filtration, the flow rate can literally need to be cut in half to achieve rated performance. Or looking at it another way, a UV Sterilizer/Clarifier rated for a 100 gallon aquarium [or pond] might only be effective for a 50 gallon aquarium!

The problem is water turbidity is not as readily measured, thus is not generally included in formulas.

The best way to ensure your water is optimally pre-filtered is to make sure to run the UV Sterilizer after a good filter, including a simple pump pre-filter such as using the AAP Filter Max.

Nevertheless most UVs now sold via discount websites as well as brick & mortar stores do not include much of anything when it comes to pre-filtration. When combined with low output medium Pressure lamps and poor design such as the gap between the UV lamp and quartz sleeve and the containment wall; the result is a UV Sterilizer with as low as 1/4 OR LESS of what many formulas rate for flow rate!

Other factors include:

A water temperature between 20 C (68 F) and 40 C (104 F)

A gap of 3 cm or less between the bulb or quartz sleeve and the wall of the unit

A reasonable dwell time, as most low cost, filter equipped UVs, and submersible UVs have poor dwell time

A GOOD water flow pattern inside the aquarium or pond.

A clean Quartz sleeve/lamp

The above is quoted from this resource: UV Sterilization; Flow Rate & Turnover Rate Table 

There is #Clarification, #Level 1, and #Level 2

Clarification is about 40-65 gph per watt with a good to excellent UV Sterilizer using a HO low pressure UV lamp.

This achieves Green water control in both Aquarium and Pond. This is the most popular application for ponds.

Level 1 Sterilization is about 20-35 gph per watt with good to excellent UV Sterilizer using a HO low pressure UV lamp [many poorly designed Chinese UVs with low output UVC lamps would bee to be slowed to a 10 gph or just to achieve level one sterilization].

This helps with Bacteria and some Virus's. Level One is the most common & recommended aquarium application.

Level 2 Sterilization is about 8-12 gph per watt good to excellent UV Sterilizer using a HO low pressure UV lamp [not at all possible with most commonly sold discount UVs coming out of China & sold via Amazon & eBay].

This helps with Parasites and “Stubborn” Viruses. This is recommended for Swimming Pools (albeit generally impractical) and similar applications.

These flow rates for each level are also impacted by water turnover, UV-C Penetration, among other factors.

For economy UV sterilizers with poor dwell time, or using medium pressure UV lamps which produce only 25% of the needed UVC irradiation, the flow rates required can be as much as 4 to 8 times lower which is why most of these $60 or less UVs commonly sold via discounters are really only good for clarification, NOT sterilization.

See this article section for more: Flow Rate & Turnover Rate Table

These flow rates can vary greatly by UV Sterilizer design, with many poor quality UV Sterilizers not being capable of level 1 or 2 sterilization at all, this is particularly common with the many "Hang-On" and submersible models sold for under $60 usd. 

Levels achieved by a UV Sterilizers are depending on the quality/design of the UV, wattage of UV, and flow rate created by a pump. The correct pump for your tank/pond size and correct sterilizer are required for accurate sterilizing levels. Ball Valves can also be used to slow water flow if pumps gph is higher than required. This is to achieve optimum flow.

See also this resource for more: Pump Specifications

For ball valves, here is one source: Aquarium Plumbing Parts; Ball Valves 

Yes and No

The use of a UV Sterilizer will NOT directly affect ammonia, nitrites, nitrates, GH, pH, KH; so the use of a UV Sterilizer should not be considered as either a positive or negative to affect these parameters.

However a properly installed and well designed UV Sterilizer will lower organics in the water column, so this can indirectly affect KH levels.

As well the use of properly installed and well designed UV Sterilizer can more directly affect the Redox Balance by reacting with oxidants in the water column, which in turns lowers oxidizing stressors to fish or other inhabitants.

A UV can also lower the level of many antibiotics in the water during treatment, so turning the UV Sterilizer off during treatment is often best. 

If a Aquarium keeper has a general knowledge of sterilizers and has overcome the first set-up of their UV Sterilizer, it should become very easy to maintain. Other than replacing UV bulbs every 6 months (& possibly cleaning the quartz sleeves if part of the UV), there is no real maintenance that is required.

If a UV sterilizer is set up correctly, it will clear up your aquarium/pond in a matter of hours/days and no over action from the keeper is needed.

See this web page for high efficiency, true UVC Bulbs: High Output UV Bulbs Page 1 from AAP 

Simply put, there are three categories of UV Sterilizer:

Category A, which have the highest dwell time, the highest UVC efficiency and output, and often are the longest lived too. These UV Sterilizers can most readily 'hit' level one or two sterilization and do so with higher flow rates than a Category B UV.

Unfortunately few UVs now sold are Category A, which was not the case a couple decades back, but with social media, discounters, etc., the lower cost Category B & C units have come to dominate.

Examples of Category A UVs include the AAP/TMC Vecton & Advantage UV Sterilizer and the Aqua Ultraviolet UV Sterilizer [the AAP being the better value].

Category B, these are still a "True" level one or even sometimes level two capable UV. These UV Sterilizers simply take lower flow rates to do so, often resulting a highest wattage unit to do the same job as a Category A. These Category B UVs also tend to me much less durable

There are still a few Category B available. Examples of Category B UVs include the AAP/TMC Terminator & Submersible UV Sterilizer, Tetra, and the Coralife Turbo Twist [the AAP again being the better value].

Category C, These are now the dominant UV Sterilizer on the market now and in fact these are not True UV Sterilizers, rather UV Clarifiers since they basically have no real ability to 'hit' level one or two sterilization, rather just Clarification.

Examples include the popular Aquarium Killing Machine, AquaTop Hang on Back UVs, most non upgraded Chinese made UVs sold by discounters, and most all submersibles. 

The simple answer is NO, but this must be qualified in that this applies to a healthy, cycled aquarium.

Even "good" aerobic bacteria found in your bio filters can be a problem when bio loads or other factors cause a "bacterial bloom" in the aquarium water, resulting in a white or grey "cloud". A UV Sterilizer will kill these bacteria when in the water column (free floating).

However in a healthy established aquarium, aerobic nitrifying bacteria should and will be found in the bio filters, substrate, live rock, etc.; NOT in the water column. Therefore, it is impossible for a UV Sterilizer to remove these beneficial bacterium. 

As with the above question, the simple answer is yes.

However if you are seasoning 'dead' rock such as so-called live rock that was just imported or you are using sterile/quarried calcium based rock (or Aragocrete), it is best to turn off your UV for a few weeks.

Once your aquarium and live rock is established, the nitrifying and de-nitrifying bacteria will and should not be in the water column, only in your live rock. So UV Sterilization is safe and in fact should be used 24/7 other than turning off for a couple hours when dosing with live products such as live planktonic algae or infusoria.

See these resources:

Aragocrete; Homemade Live Eco Friendly Rock

More UV Sterilizer Questions, Myths Answered 

This depends upon the design, but in general the answer is an emphatic NO, at least with all under $60 usd units often sold by Amazon or eBay!

The problem is the chamber where the UV-C irradiation/sterilization reacts with the water often has too high a flow rate along with too large a gap between the UV wall and lamp/sleeve, resulting in poor dwell time.

See: Water Flow, Dwell/ Exposure Time; UV Sterilization Levels

As well, many of these Submersible UV Sterilizers have a very short lifespan due to ballast failures (one, the Killing Machine even requires replacement of the ballast when replacing the UV Lamp). Unfortunately most online reviews (Amazon, etc.) do not reflect expert long term usage and only report initial results.

Finally Amazon and other bargain sellers will sell similar models [such as by SunSun], but these are stripped down, without essential pre-filters needed for lowering water turbidity for higher UVC efficiency, as well these models are NOT adjusted for correct UVC flow rates.

AVOID these low cost Hang On UVs, Lift Tube "Category C" UVs, and small self powered UVs often peddled by those in the aquarium industry that give the industry a "black eye".

About the only only reasonably effective submersible UV Sterilizer is the AAP CUP Series 13 Watt, along with a few other exact duplicates that include adequate pre-filtration, a low pressure HO UV lamp, and the correct flow rate.

Even then these should only be considered an economy UV Sterilizer for those on a budget and desiring simplicity (these still cost more than the non level 1 sterilization models previously noted).

See this product listing: CUP Series Submersible UV Filters from AAP

Recommend further reading; References:

*Internal, Submersible UV Sterilizer Filter, Pump; Review

*Green Killing Machine, Submariner UV Review

*UV Sterilizers Gimmicks; 'Turbo' Twists, Baffles Wipers 

As noted in the last section, an inline UV Sterilizer is best. This should be installed after your filter as the less particles your UV has to get through the better results you will have. Also, installing your UV after your filters allows the filters to have the best possible bio filtration.

See:

UV Sterilizer Examples 

While there are several well designed UV Sterilizers available, most are often far more expensive than the low quality UVs that produce little or no level 1 sterilization of which these should be avoided.

A simple tip would be to avoid ANY UV Sterilizer under $60 (US dollars), none of these have even close to adequate dwell times to perform level 1 or level 2 sterilization.

See: UV Sterilizer Dwell Time Test

For the best true level 1 and/or 2 UV Sterilization with high dwell time, your best bet are the AAP/TMC Vecton and Advantage UV Sterilizers among a few others such as Aqua UVs.

See this product listing for a professional site selling these:

AAP/TMC UV Sterilizer, Premium High Dwell Time UVC Sterilization

The AAP Compact UV is a "good" UV Sterilizer, and in fact among the best of the "Compact" UVs, although the popular compact design is not as good a design as the high dwell time straight tube such as the AAP/TMC. Other good compact UVs includes the Tetra, albeit over priced.

Beware of the Jebo, AquaTop, and Turbo Twist, and the plethora of low quality so-called UVs sold on Amazon.com, as despite good marketing, these have poor life spans or little to no level 1 sterilization as per many professionals who have used these units and were consulted for this article.

It is unfortunate that many discount UV Sterilizers sold via Amazon, eBay, etc.; even with the same model model number come with low output medium pressure low output UV lamps/bulbs that have only 1/4 the output of a low pressure HO UV bulb/lamp. What many do not understand is that as a retailer or distributor, one can order these UVs from China at a lower/cheaper price by getting a stripped down version.

The end result is a UV or replacement lamp that is good for clarification at best and even then not very good at this either.

See the next section about UV lamps/bulbs for more, including excellent references!

See this product listing: AAP UV Sterilizer with HO UVC Lamps 

Generally speaking for optimum Sterilization the UV Bulb should be changed every six months for an aquarium.

For a pond every 12 months in cool climates and six months for warm climates. 

As for the replacement bulb itself, this can be a bit more difficult since it seems everyone and their brother are now selling these on the internet [including Amazon & eBay], as well as some Home Improvement Stores.

However many, if not most of these bulbs sold at these locations are NOT true low pressure, hot cathode UVC bulbs which provide the optimum output and efficiency, even though one eBay seller in particular has taken to copying the information from another online seller that sells true high output UVC bulbs/lamps.

This is especially problematic with many of the Compact UV Bulbs now sold such as the 5, 9, & 18 Watt UV bulbs, lamps which are sold for less than it cost to produce the better quality true UVC bulbs.

If the price seems too good to be true such as $5.99 or even $9.99, you are likely getting a "nail curing" UV lamp, not a true high output low pressure UV lamp/bulb, despite what the sellers eBay or Amazon listing might say.

A quick comparison of the UV Bulbs/lamps reveals the difference.

Reference: 9 Watt UV Replacement Bulb/Lamp Review

The diagram/picture above demonstrates why is is so important to purchase the correct UV replacement bulb/lamp.

Even many new low cost so called UV Sterilizers, which in reality are at best clarifiers are supplied with these lower cost medium pressure UV Bulbs. With the cost just to produce a quality low pressure UV Bulb, chances are ANY UV Sterilizer advertised new for under $60 is going to contain these lower output bulbs.

What to look for?

Price for one, you cannot get a true low pressure hot cathode 9 watt UVC bulb for under $10 (or likely under $12). When the manufactured cost of a HO 9 Watt lamp is $10 usd, a retailer CANNOT sell these for $10 or even $12

Does the seller even claim that their bulb is a low pressure hot cathode UVC germicidal bulb/lamp?

The 9 Watt and 18 Watt UVC germicidal bulbs/lamps now can be found with a patented metal heat shield, which among other things lowers the amount of energy lost to heat and thus increases efficiency. These though can be hard to find in most locations, but are well worth looking for and purchasing due to their extreme high efficiency.

Or for sources to purchase a TRUE low pressure, hot cathode UVC Bulbs:

#High Output Compact UV Replacement Bulbs/Lamps (such as the popular 9 Watt HO UV Replacement Bulb)