A UV sterilizer emits ultraviolet light rays into your aquarium water which can effectively damage the DNA of harmful bacteria, parasites, and algae which then reduces the proliferation of these pests in your aquarium. When set up correctly an Ultraviolet Sterilizer can improve water clarity, slow the spread of algae, reduce dinoflagellates, and prevent outbreaks of disease and parasites.

The challenge for many hobbyists is understanding how to install and use the UV sterilizer correctly and achieve the most effective sterilization because if you're not doing it right, the benefits are negligible at best. 

How a UV Sterilizer Works

While UV sterilization for aquariums has been around for quite some time, it is only been the last 10-20 years or so that reef hobbyists have adopted this method. Early skepticism revolved around the fact that UV sterilizers were thought to disrupt the biodiversity and natural food chain that was critical to reef aquariums but this has been proven wrong time and time again. The reality is that you can have an incredibly biodiverse reef aquarium with a healthy biological filter and still run a UV sterilizer. 

UV Sterilizer Diagram

The way it works is quite simple, aquarium water is pumped through a chamber that contains ultraviolet light. The ultraviolet rays damage the DNA of living organisms that are suspended in the water making it impossible for those organisms to reproduce. This can effectively keep the population of harmful pathogens and algae to a minimum and, therefore, reduce or eliminate the effects of those pathogens in your aquarium.  

In order for the UV sterilizer to be effective, the pathogens must be suspended in the water and pass through the UV with sufficient exposure to ultraviolet light.

Pentair 40 watt UV Specifications

Different organisms can tolerate different levels of UV exposure which is why we adjust the flow rate to target specific types of organisms. The slower the flow, the longer the exposure to UV light rays. Then again, if you go too slow, some pathogens can reproduce faster than you can sterilize them so it's a balance. Finding that flow balance to target specific pathogens is the key to success. 

For the most part, you can choose one of two different flow rates as recommended by the manufacturer when tuning your sterilizer. See the specifications above for the Pentair 40 Watt UV. The higher flow rates are used to target bacteria and algae while the slower flow rates are used to target protozoa (disease-causing parasites). 

Applications for UV Sterilizers

UV sterilizers can be applied all the time or just periodically to address a problem.  When used correctly, they can help overcome things like dinoflagellates and slow down the spread of algae.  You might also use a UV when stocking the aquarium to help reduce the chances of disease.  Some hobbyists choose to run them 24/7 while others choose to run them on a timer for only a portion of the day. Some folks just keep a UV on hand and will only use it when something comes up, the Aqua UV Advantage hang-on sterilizer is great for this just because it's small and can hang right on your display.  

The most important thing to remember about UV sterilizers is they are not killing pathogens or eliminating them completely but rather reducing the free-floating population in your aquarium which then reduces the negative effects. Anything that doesn't suspend into the water column will not be harmed including beneficial bacteria. 

How To Choose a UV Sterilizer and Pump

UV Sterilizers are rated via wattage. Larger tanks require higher wattage UV sterilizers and simply use the manufacturer's recommended tank ranges. Be mindful of the UV sterilizer's physical size and where you need to mount it; they can be cumbersome and awkward to mount inside your tank stand. 

Choose a pump that can supply the recommended flow rates outlined by the manufacturer, be mindful of any head pressure that will be created via the plumbing, and always install a valve so you can control that flow rate. Controllable DC pumps will work just as well for controlling the flow but the most important thing is to ensure you can achieve the recommended flow ranges for your UV sterilizer. 

Most UV sterilizers will not include a pump or any of the required plumbing so you will likely need some tubing, hose clamps, and possibly some plumbing fittings to get the job done. The exact fittings and tubing would be based on the particular specifications of the UV sterilizer you choose and it is best to simply adapt to the tubing size of the pump. 

How To Install A UV Sterilizer

1. Mounting Configuration

Find a suitable location to mount your sterilizer within close proximity to your sump or aquarium. When mounted horizontally, the inlet and outlet should be facing upwards to avoid trapping air.  When mounted vertically, the water should enter through the bottom and exit through the top of the UV sterilizer for the same reason. Trapped air can heat up and reduces the effectiveness of the sterilizer. Most UV sterilizers can be mounted on the wall using specialized brackets or PVC clamps.

2. Pump Location

A separate pump is used to push water through the UV sterilizer and it is best to use a dedicated pump to have absolute control over the flow-through rate. 

You need to set up the UV sterilizer in such a way that prevents recycling, pull water from chamber A and return that water to Chamber B or C downstream.  If water is returned from the UV into the same chamber it is fed from, it will recycle the same water and reduce the effectiveness of the UV. 

UV Sterilizer Installation
  • The best configuration is to place the pump into the first or second chamber of your sump, then return the water into your final return pump chamber so it is immediately pumped back up into your display.
  • Alternatively, your could pull water from the sump and return it directly to your display.
  • If you do not have a sump, the water will be pulled from and returned to the display. 
  • AIO aquariums should follow the same rule where water is pulled upstream and returned downstream in the filtration. 
  • Setting up a closed-loop directly off the display is also acceptable and is the most effective installation; it's just not the most convenient. 

Aqua UV makes the Advantage HOB UV Sterilizers which are designed for hang-on your display and are suitable for tanks up to 75 gallons. These work out great for smaller aquariums or situations where you don't want to plumb a full-size UV and are incredibly easy to set up.

3. Connecting the Pump

Flexible tubing is the easiest route to connect the pump but PVC is also suitable. Reference the inlet/outlet connection size of your UV sterilizer and then adapt that to fit the tubing size of your pump. Use hose clamps to prevent links and secure the return tubing in such a way that it is not at risk of falling out of the tank/sump when running. 

4. Install the Bulb

Installing the UV lamp is easy, be sure the UV sterilizer is not plugged into the wall and unscrew the compression cap on the end. Plug in the lamp/bulb using the 4-pin connector and take care not to touch the glass on the bulb directly with your fingers, wear gloves or use a towel when handling the bulb. Do not apply power until water is moving through the UV to avoid heat build-up and damage to the UV. 

4. Tune the Flow Rate

Finally, it's time to turn on the pump and dial in the flow. Flow rate is critical and choosing the right pump can get you into the ballpark.  That said, measuring the flow is the only way to know for certain and there is just a couple of ways you can do that.  We also highly recommend the video linked at the bottom of the page for more detailed tuning instructions. 

  • The bucket test is sort of the analog way to do things but is also the easiest; simply time how long it takes to fill up a 5-gallon bucket using the UV pump. This will tell you how fast the water is moving through the UV and you can then adjust the flow accordingly to hit your desired flow rate.
    • For example: If it takes 1 minute to fill a 5-gallon bucket with water, the pump is pushing 300 gallons per hour.  If it takes 2 minutes to fill the 5-gallon bucket, the pump is pushing 150 gallons per hour.
  • Alternatively, you would need to plumb an electronic flow meter. Neptune Systems Apex users have access to the FMK - Fluid Monitoring Kit which is a great option but limited to users of the Neptune Systems Apex controller.