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During my 30 years as a fish keeper, breeder and showman, I have tried many different pieces of equipment, all of which were designed to improve the health of my fish. Some pieces did, others didn’t. UV sterilizers are one of those pieces of equipment that had a real positive effect on my fish’s health and well-being.
UV sterilizers are small pieces of electrical equipment that sit either inside or outside the aquarium (depending on the model). Water is passed through the sterilizer and over an ultraviolet bulb. The ultraviolet light kills bacteria and pathogens and causes free-floating single-celled algae that cause green water to stick together, allowing your filter to catch them.
To help others decide where to spend their money, I put 4 of the best UV sterilizers to the test to find which offers fish keepers the best value for money.
What Do UV Sterilizers Do?
UV Sterilizers (UV standing for Ultraviolet) are devices that can be located either inside (internal) or outside (external) the aquarium depending on which brand is used. Over the years I have tried many different UV Sterilizers, both internal and external, and I have found both styles to be more or less ass effective as one another.
UV sterilizers work by passing aquarium water over a bulb that emits ultraviolet light. The ultraviolet light kills bacteria and pathogens that a present in the aquarium water. The light also causes single-celled algae that cause green water to stick together, allowing the filter to remove them from the water.
One of the major bonuses to using a UV sterilizer to kill bacteria rather than chemicals is that ultraviolet light does not have an effect on the aquarium inhabitants, whereas chemicals can have an adverse effect on fish, shrimp, snails, and even plants.
How Do UV Sterilizers Work?
UV sterilizers are simple in the way they work. As water from the aquarium passes through the UV sterilizer, it travels across a bulb that emits ultraviolet light. As the ultraviolet light passes through the water, it hits any bacteria, pathogens, or algae cells that are free-floating in the water.
As the UV light passes through the single-celled organisms it alters their cellular function meaning the bacteria or pathogen is either killed or becomes unable to reproduce. If the bacteria or pathogen can not reproduce its number within the aquarium quickly dwindle until they die out completely.
By either killing the bacteria or pathogen outright or removing its ability to reproduce, the problem they were creating disappears.
When the ultraviolet light hits the single-celled algae that cause green water in our aquariums, the algae cells stick together. As single-celled algae are normally too small to be caught in the filter, they just pass through, causing green water. After the ultraviolet light makes the algae cells stick together they can be caught in the filter sponges or floss.
Do UV Sterilizers Kill Beneficial Bacteria?
I have given many talks at fish clubs around the country about filters and sterilizers, and one question that crops up frequently is do UV sterilizers kill beneficial bacteria? The answer is no, UV sterilizers do not kill beneficial bacteria in an aquarium. Beneficial bacteria are not free-floating in the aquarium, they attach themselves to every surface, including filter media. As the UV light only affects the water flowing through it, it does not kill the beneficial bacteria living in our filters.
There is however an important point to note. When establishing a new aquarium, the bacteria are initially free-floating in the aquarium water before they establish a colony in the filter. Running the UV sterilizer before the bacteria have had a chance to establish themselves can seriously slow down the cycling of the aquarium.
Benefits of Using UV Sterilizers
There are many benefits to using an ultraviolet sterilizer in an aquarium. In my experience, the main advantage of using an ultraviolet sterilizer is it helps us cut down the number of chemicals we need to use to keep our fish healthy.
There are countless occasions we have to add chemicals to our fish tanks. One of the major reasons is when we have Ich (Whitespot) present. Ich is a tiny parasite that lives in the water column where it looks for a host fish. Once it finds a host fish, the Ich parasite burrows under the fish’s scales where it feeds on the fish’s blood for several days before dropping off. The parasite then multiples and each of the new Ich parasites go in search of a host fish.
When we have a UV sterilizer running on our aquariums, any Ich parasites present in the water that passes through the UV sterilizer will be killed.
Further benefits can be felt when our aquariums are suffering from green water. Green water is caused by single-celled free-floating algae that multiply rapidly in our aquariums, causing the water to turn green. These single-celled algae are too small to be collected by most aquarium filters, which is why we end up with aquarium water that looks like pea soup.
When green water passes through a UV sterilizer, the ultraviolet light causes the algae to stick together, which means it is easier for our filters to pull the algae out of the water, resulting in clear water. Many aquarists only run UV sterilizers when they are suffering from green water.
We in the freshwater fish keeping hobby have been slow to take up the use of UV sterilizers, whereas many of our friends in the marine hobby wouldn’t dream of running their tanks without a UV sterilizer attached.
Are UV Sterilizers Safe For Fish and Shrimp?
UV sterilizers are completely safe for use in tanks with fish, shrimp, snails, and plants. In fact, using a UV sterilizer is far less harmful to the tank inhabitants than adding chemicals to the water.
Whether you use an internal or an external UV sterilizer, the fish, shrimp, snails, and plants do not come into contact with the UV light. The unit is self-contained.
In the extremely unlikely event, a fish or shrimp physically pass through the UV sterilizer then the effects of the UV light may be detrimental, but the reality is, to get into the UV sterilizer, the fish or shrimp would have to have passed through an impeller, which would cause far more damage than the UV light.
UV Sterilizer Considerations
Once you decide a UV sterilizer is for you, there are a number of different options that need to be considered prior to selecting which model is right for you.
What size sterilizer to use?
When deciding which size sterilizer you will need, there are two parameters to consider. These are;
We can use these two pieces of information to perform a simple calculation that allows us to work out if the UV sterilizer in question is powerful enough for our tank. Ideally, a UV sterilizer should be powerful enough to turn the aquarium water over one to two times every hour. If the unit is less powerful it may not effectively kill all the bacteria or pathogens living in our water columns.
The calculation we perform is as follows.
The number of watts of the UV bulb times gallon per hour of water turnover divided by 1.5
For example, if a UV sterilizer has a 15-watt bulb and turns over 30-gallons per hour, the calculation would be as follows;
15 x 30 = 450 / 1.5 = 300
This means the unit in question would work for an aquarium up to 300-gallons (1,363 liters).
What flow rate is right?
Determining the right flow rate for your aquarium can be tricky. The higher the flow rate, the more times the aquarium water will be passed through the UV sterilizer per hour, but the less time the water will be exposed to the UV light on each pass (known as dwell time).
In theory, the higher the flow rate, the more powerful bulb will be required.
One thing to be aware of, when you look at the box of a UV sterilizer, or on the manufacturer’s website, the flow rate is often exaggerated. The manufacturer will normally measure the flow rate without any equipment attached, including pipework. The true flow rate or GPH (gallons per hour) may be less than that which is listed.
What is dwell time?
When reading about UV sterilizers, you may hear others discuss dwell time. Dwell time refers to the amount of time a given quantity of water is in contact with the UV light. The longer it takes for a set amount of water to pass fully across the UV bulb, the longer the dwell time.
The reason dwell time is important is because the longer the water is exposed to the UV light, the greater the chances of any bacteria or pathogens in that water being killed. Generally speaking, the longer the dwell time, the more effective the UV sterilizer has.
Most people think that dwell time is totally dictated by the flow rate. The theory being the faster the water is flowing the lower the dwell rate, and vice-versa. However, the length of the UV bulb also affects dwell time.
For example, if you have two UV sterilizers, both with a flow rate of 200-gallons per hour, but one has a 4″ (10cm) UV bulb and the other has an 8″ (20cm) UV bulb, the water in the second sterilizer will be exposed to UV light for roughly twice as long as the first unit, even though the flow rates are identical.
Are Internal or External UV Sterilizers Better?
As mentioned above, UV sterilizers are available as internal models (which sit inside the aquarium) or external models (which are located outside the aquarium).
Whether you choose an internal or external model will often be decided by what equipment you already have in your aquarium. Normally, external UV sterilizers require some sort of additional, external source of water movement, like a canister filter. An external UV sterilizer is usually fitted to the return pipe of the canister filter, subjecting the water to UV light before it returns to the aquarium.
External UV sterilizers are also often used when an aquarium is filtered by a sump.
There are some external UV sterilizers on the market that have their own pump to draw water from the aquarium, pass it through the sterilizer and return it to the tank, but these are often not popular units and they sell less frequently than other models.
Internal UV sterilizers on the other hand are normally fully stand-alone units that are placed inside the aquarium. These UV sterilizers almost always have their own impeller which draws water in, passes it across the UV bulb, and returns it to the aquarium.
Internal UV sterilizers can be used regardless of filtration arrangements. Small, internal units are often used by professional breeders who may have hundreds of aquariums in their care. They can move the internal sterilizer between fish tanks as and when required.
In my own fish room, I have several small internal UV sterilizers that I reserve only for tanks that are suffering from green water.
Are UV Sterilizers and UV Clarifiers the Same?
No, UV sterilizers and UV clarifiers are not exactly the same thing. UV clarifiers are often less powerful than UV sterilizers, and whilst both devices will clear up green water, UV clarifiers are seldom powerful enough to kill bacteria and pathogens in the water.
If you are only looking to fit UV sterilizers due to green water, then a UV clarifier will work well. To my mind, however, if you are going to spend the money on a clarifier, spend a few dollars more and buy a UV sterilizer. The extra cost will be insignificant if you get an Ich outbreak in your aquarium.
How Often Should UV Bulbs Be Changed?
Some manufacturers specify how often a bulb should be changed, others don’t. There are many factors determining how often a UV bulb should be changed.
Unlike regular lightbulbs that we use around our house, UV bulbs often need changing even though they appear to still work. We don’t normally wait until the bulb blows or physically stops working before we change them. Over time, UV bulbs become less effective. The longer they have run the less UV light they give off.
As such, UV bulbs will normally need changing about every 12 months. Assuming you run your UV sterilizer 24 hours a day, 365 days a year, it will run for almost 9,000 hours. 9,000 is often quoted as the expected life of a UV bulb.
In my own fish room, I change all the UV bulbs on the same day each year. I try and keep them all in sync, otherwise, I could never keep track of which one was changed when. No matter when I buy another UV sterilizer, the bulb will be replaced on the 1st of March every year. It’s UV bulb day in my fish room!
Are UV Sterilizers Good For Freshwater Aquariums?
In my experience, yes, UV sterilizers are good for freshwater aquariums. In my fish room, I only run freshwater aquariums, and I have used UV sterilizers to keep many aquariums clear of green water, Ich, and many other bacterial infections.
How Long Should UV Sterilizers Run For?
How long a UV sterilizer runs may depend on why you are using it. When I use my internal UV sterilizers to get rid of green water, I only run the units for 3 or 4 days, or however long it takes to clear the aquarium of the green water. After which time I take the unit out and store it ready for next time.
On my Discus display tanks and some of my guppy breeding systems, I run the UV sterilizer 24 hours a day 7 days a week, all year round. I want the water in those tanks to be free of bacteria and pathogens at all times. The stock in those tanks is worth many $1,000s to me.
There are a good number of fish keepers who may only run their UV sterilizers when they have an outbreak of Ich or another pathogen that needs to be eliminated.
UV sterilizers do not need to run all day every day in an aquarium, but they can. It’s very much a personal choice.
Do UV Sterilizers Require Maintenance?
As with any piece of equipment in the aquarium, UV sterilizers do require maintenance. However, that maintenance is minimal and the intervals between maintenance are long.
In my experience, to keep a UV Sterilizer in top condition, you need to carry out the following, usually about once every 6 to 12 months.
I have found following this routine has kept my UV sterilizers working to maximum efficiency.
The Best UV Sterilizers
So, having looked at how UV sterilizers work and why we might want to use them with our freshwater aquariums, let’s have a look at which units I feel are the best ones to choose from. To determine which UV sterilizers are the best, I have tried and tested each unit listed below. Some are internal and some are external UV sterilizers.
In deciding which ones are best I have considered how easy they are to use, how effective they are when in use, and how easy they are to maintain. I have put 4 units to the test to give the following results.
1. Fluval UV Clarifier
The first UV sterilizer on my list is actually classed as a UV clarifier. This clarifier is primarily used to rid aquariums of green water.
The Fluval UVC In-Line Clarifier has been designed to work with Fluval’s range of canister filters, although I have found they work just as well with other manufacturers’ canister filters, providing the correct size hose is used. This clarifier is fitted after the canister filter, meaning as the water leaves the canister filter via the return hose, it passes through the UV clarifier.
In my experience, the Fluval clarifier does a great job of keeping the water clear of free-floating algae and I don’t have green water in any of the aquariums I run this clarifier in.
This unit is really easy to install. You simply cut your return hose from your canister filter using a sharp knife, and insert one end of the cut hose into the UV clarifier inlet and the other piece of hose into the outlet, then start the canister filter up.
The UV clarifier sits there (or screwed to the cabinet in my case) and purifies the water.
Fluval has designed this clarifier so the water runs along the length of the bulb in one direction, then back up the bulb in the other direction, effectively doubling the dwell time.
There is one issue I have discovered with these units, and I didn’t realize it until after I bought some. You can not change the UV bulbs. Fluval says the bulb will work for around 30,000 (which is just over 3 years if left on 24 hours a day) after which time the whole unit will need to be replaced. So far I have not replaced any of mine.
The Fluval Inline Clarifier is not the cheapest unit on the market. Mine cost me around $90 each, but so far they have proved to be worth the money.
2. Aqua UV 15 watt Advantage 2000+
The Aqua UV 15 watt Advantage 2000+ is one of my favorite UV sterilizers because of its portable nature. This sterilizer has been designed to hang on the side of an aquarium, meaning you can attach it to the return pipe of your canister filter, or use it as a standalone unit with a small submersible pump.
Personally, I use one of these UV sterilizers with its own submersible pump, making a sterilizer I can move from tank to tank as and when required. I often use this sterilizer in my quarantine tanks, perhaps running it for a week on one tank before transferring it to use on another tank.
The top of the unit has a return spout which allows water to return to the tank. This spout doubles up as the hanging hook, attaching the unit to the side of the aquarium, which I have found to be an excellent design.
Truth be told, the is a fairly expensive unit, coming in at close to $200.
Unlike the Fluval Clarifier above, this device is a proper sterilizer and the bulb is powerful enough to kill bacteria and pathogens floating in the water.
Aqua Advantage UV sterilizers come in two different sizes, 8-watt and 15-watt. The 8-watt unit has been primarily designed for freshwater aquariums up to 200 gallons (757 liters) when used as a sterilizer or 1500 gallons (5678 liters) when used as a clarifier. The 15-watt unit has been designed for freshwater aquariums up to 500 gallons (1892 liters) when used as a sterilizer and 2000 gallons (7570 liters) when used as a clarifier.
I have found this sterilizer is excellent for use in aquariums that only have internal filtration, such as sponge filters.
It is fairly easy to swap out the UV bulb on this unit. This unit is made in the United States.
3. Coralife Turbo-Twist UV Sterilizer
The Coralife Turbo-Twist UV Sterilizer is one of the go-to UV sterilizers in my fish room. I use several of these units.
The main reason I find this UV sterilizer so good is the dwell time. As the aquarium water passes through the sterilizer, the spiral design means the water passes around and around the UV bulb several times, creating the maximum exposure time resulting in a high rate of destruction for any bacteria in the water as well free-floating single-celled algae, preventing green water.
I have 2 or 3 of the UV sterilizers mounted in cabinets under my aquariums and a couple of them hooked directly onto the glass, acting as a return spout. This small feature really increases the adaptability of the sterilizer.
The Coralife UV Sterilizer is an 18-watt UV lamp that the manufacturers say has a work-life of around 8,000 hours, which for me means it needs changing about every 12 months. I like the fact these bulbs can be changed with any tool. The cap of the unit twists off and the bulb can be quickly replaced.
This sterilizer is rated for aquariums up to 250-gallons (945 liters) and it is recommended that a flow rate of between 200-400 GPH for maximum efficiency.
4. Sunsun Jup 01 Internal UV Sterilizer
This UV sterilizer is a true plug-and-play unit that simply has to be placed into the aquarium and switched on. It is fully self-contained and doesn’t require any additional pumps or powerheads to work.
I like to use these UV sterilizers in my quarantine tanks because they can be quickly taken out of one tank and placed into another. The small but powerful 8-watt motor moves around 210-gallons per hour. Most of my quarantine tanks are only small, but this unit could be used in much larger aquariums.
One important point to remember with this, and all internal UV sterilizers, is when you transfer the unit from one aquarium to another, do take time to make sure no air is trapped in the sterilizer before turning it on.
Because this unit has its own built-in pump, it can be used no matter what filter you are running on your aquarium. Again, going back to my quarantine tanks, they all run sponge filters, so I could not run any of the external units that need to be connected to a canister filter.
I find these UV sterilizers really good, and I have recommended them to several of my fish-keeping friends. At around $50 I think this UV sterilizer is priced well and affordable for most hobbyists.
UV sterilizers are a great addition to our aquariums and they are by far the easiest way to control both free-floating bacteria and pathogens, as well as single-celled algae that cause green water.
Choosing the right sterilizer for your size aquarium is important, but with a bit of research, it is simple to work out which device you need.