The Salty Side: Saltwater Aquariums for Beginners

UV Sterilizer in a Saltwater Aquarium - Are They Worth It?

The decision to add a UV sterilizer in a saltwater aquarium is usually determined by a sudden need for one. You may have inadvertently introduced Cryptocaryon irritans (white spot), or you suddenly have Dinoflagellates inundating your display. You may however, be thinking of using it as a prophylactic method of control, to limit the potential for unwanted algae or parasites. We look at the benefits and dispel misconceptions in this article.

uv sterilizer in a saltwater tank

What does a UV Sterilizer do in a saltwater aquarium?

A UV sterilizer is simply a black plastic tube with an ultra violet light inside a crystal sleeve secured through the centre. Water is pumped into the sterilizer from one end, usually passing through an inner spiral design, and exits the tube at the other end. A UV sterilizer uses an independent water pump to direct water through it. The water that passes through the sterilizer comes into contact with ultra violet rays emitted by the UV bulb. Bacteria, algae and parasites are intolerant to UV rays, dependent on contact time with the light and concentration of the emission, and are consequently killed.

How long should a UV sterilizer be on in a saltwater aquarium?

The purpose of running a UV sterilizer is to eliminate existing or potentially unwanted parasites, bacteria or algae. Unfortunately, they do not go on vacation. It is really in your best interest as a reefer to run your UV sterilizer 24 hours a day, 7 days a week. Considering the question in a little more depth, algae such as dinoflagellates are known to spread in the night. Tomites, the waterborne stage of the white spot parasite also emerge in the dark. However, green algae proliferates in the day. With these in mind, it is well worth keeping your UV steriliser on at all times.

Coralife Aquarium Fish Tank Marine Salt Water Turbo-Twist UV Sterilizer 6X, For up to 250 Gallons
  • Removes free floating harmful micro organisms from aquarium water
  • Features a unique twist flow design increases water’s exposure to ultraviolet light improving the treatment of unwanted microorganisms
  • For fresh or saltwater applications

How fast should the water flow through a UV sterilizer?

This is known as contact time. To ensure that waterborne parasites or algae are killed when passing through the steriliser, they need to be in contact with the emitted light for a specified time period. This period of time depends on what you are striving to eliminate, or control. Contact time is controlled by the rate of water flow from the pump you are using.

Will a UV sterilizer help with ICH, also known as white spot?

Cryptocaryon Irritans, the parasite responsible for the infection that looks like your fish have been sprinkled with salt, can be controlled with the introduction of a UV Sterilizer. The life cycle of this parasite includes a stage where they become waterborne, they are known as theronts. It is at this stage where they are actively searching for a host. If you have an adequate UV sterilizer in a saltwater aquarium, these theronts will inadvertently find themselves being drawn through the a killing spectrum of UV light.

The suggested radiation intensity to combat Whitespot is 72,000 microwatt’s/sec/

What this means is that in order for your UV to work, you will need to adhere to the minimum requirement of 1 Watt for every 10 gallons* of water to sterilize Cryptocaryons irritans. However, the larger wattage you can go the better.

Using the above criteria, the following wattage UV sterilizers have been paired with the correct aquarium volume.

Aquarium size – 50 gallons – 5W UV sterilizer100 gph pump
Aquarium size – 90 gallons – 9W UV sterilizer180 gph pump
Aquarium size – 180 gallons – 18W UV sterilizer300 gph pump
Aquarium size – 360 gallons – 36W UV sterilizer 550 gph pump

In a reef tank, you want to pass the volume of the aquarium through the UV 1 to 2 times an hour.
In a fish only setup, you want to pass the volume of the aquarium through the UV 2 to 4 times an hour

Coralife Aquarium Fish Tank Marine Salt Water Turbo-Twist UV Sterilizer 12X, For up to 500 Gallons
  • Removes free floating harmful micro organisms from aquarium water
  • Features a unique twist flow design increases water’s exposure to ultraviolet light improving the treatment of unwanted microorganisms
  • For fresh or saltwater applications

Will a UV sterilizer kill dinoflagellates?

Dinoflagellates are a type tenacious algae that take advantage of limited tank maturity, or unbalanced saltwater aquariums, to become persistently hard to irradiate. Fortunately, some strains of dinoflagellates are easily knocked down and killed with the use of the correct wattage UV sterilizer.

At the time of writing this article the eradication of of dinoflagellate using a UV sterilizer had not been tested in a lab, or verified in papers. However, within the hobby it was found that dinoflagellates such as Ostreopsis could be swiftly killed when done correctly.

To be effective, a UV sterilizer should be run at 1W per 3 gallons, however, a larger UV sterilizer would be better. Therefore a 90 gallon tank would need a 36W UV sterilizer. The flow rate through the sterilizer, for it to kill any free swimming dinoflagellates that pass through it, should be between 65 to 100gph. For larger aqauriums the flow rate should be 90 to 200gph.

Aquarium size – 50 gallons – 18W UV Sterilizer – 100 gph pump
Aquarium size – 90 gallons – 36W UV sterilizer – 100 gph pump
Aquarium size – 180 gallons – this will require a large UV sterilizer of 60W or more

Will a UV sterilizer control algae in my saltwater aquarium?

After the installation of a UV sterilizer you will notice a significant reduction in blown film algae on the aquarium glass as an indicator that it is working.
They are also very efficient at ridding algae blooms in a salt water tank. UV sterilizers are also very good at limiting the spread of green hair algae, if the filaments find their way through the tube. That is the point, however, that UV can only be used as part of the control method for algae. It will only have a direct impact on the spores or filaments that pass through.

The flow rate for algae control is normally the same as suggested on the packaging.

Just starting out in the saltwater hobby? Get your copy of How To Set Up A Successful Saltwater Aquarium ebook today.

This ebook will help to guide you past the mistakes I have made over the years.

Is a UV sterilizer bad for a reef tank?

As good as UV sterilizers are at controlling algae, bacteria and parasites, they are still rumoured to interfere with the natural food chain in your aquarium, indiscriminately killing phytoplankton and other microorganisms that may feed your fish or inverts.

This negative effect couldn’t be further from the truth. If it were to be believed then you would also be required to believe that our reef aquariums are alive with phytoplankton and waterborne organisms. Unfortunately, this is not the case. Our corals and inverts rely on the food we add, because those imagined phytoplankton and microorganisms do not exist at a level consistent enough to feed our inhabitants. Furthermore, we already employ the use of skimmers, filter socks and roller mats that do a good job of keeping the water clean. When feeding live phytoplankton it is always recommended to turn off the UV sterilizer for at least half and hour. This allows them to be gathered up by your corals and inverts. It is unfortunate that phytoplankton is unable to naturally proliferate in a saltwater aquarium, but our tanks simply cannot sustain their lifecycle.

You may also hear that a UV sterilizer can have a detrimental impact on the ‘good’ bacteria in a reef tank. When referring to ‘good’ bacteria, we are referencing nitrifying and denitrifying bacteria which are responsible the biological foundation of your saltwater aquarium. These bacteria reside primarily on and in live rock, sand and on surfaces. They do not remain suspended in the water column.

To summarise, a UV sterilizer in a saltwater aquarium can kill or sterilize whatever happens to pass through it, but it certainly does not starve your reef tank or disrupt the biological foundation.



Scroll to Top