What Does a UV Sterilizer Do? Is It Worth it?
UV sterilization is a somewhat contentious topic for saltwater aquarists because the use of UV sterilization has long been riddled with myths and misinformation. It has only been very recently that we have gained access to quality information and use techniques that have proven UV sterilization to be very effective. That said, it is important to understand what exactly a UV sterilizer can and cannot accomplish to set a realistic expectation.
Select A Goal
UV sterilizers can accomplish a few different things and as a hobbyist, you have to choose which of those things you want to accomplish.
Bacterial and Algae Issues
A UV can effectively eliminate cloudy water due to bacterial blooms, kill cyanobacteria and dinoflagellates, and reduce the spread of nuisance algae in your tank. This is a pretty common desire for most hobbyists and is the best use case for UV sterilizers. They will improve water clarity across the board and help to reduce the growth of some unsightly pests but only when used properly.
In order for the UV sterilizer to kill something, it must pass through the UV sterilizer so it has to be suspended in the water. If the algae are growing on your rocks or the cyanobacteria is covering your sand bed, the UV sterilizer cannot kill it. It can only reduce the populations of those ailments that are free-floating in the water. This can reduce the rate of growth and spread, but won't kill off what is already growing on your tank's surfaces. In this case, it's a great preventative measure and should be looked at as a method of control as opposed to eradication. Combine UV sterilization with proper husbandry and maintenance practices and you're far less likely to experience a pesky outbreak.
This is one of the more debated uses of a UV sterilizer because the lack of good information has only recently become available. Protozoa is a blanket term for common parasites that can plague aquarium fish. When the protozoa pass through the UV sterilizer, they are bombarded with UV rays that break down their DNA and prevent the parasites from reproducing. This reduces the overall population of the parasites in your aquarium which means your less likely to experience a widespread parasitic infection.
Again, this doesn't mean you can kill what has already infected your fish and should be looked at as a method of prevention and control. When combined with proper quarantine protocols and sourcing/keeping healthy fish, you can dramatically reduce the chances of experiencing these infections, but you will never eliminate that risk altogether.
Proper Implementation of UV Sterilization
The goals we defined above can only be accomplished through specific installation and tuning of the UV sterilizer. First and foremost, you have to choose the right wattage for your tank size no matter what your desires might be. Next, you have to install the UV sterilizer properly to that you can effectively sterilize your entire body of water. Finally, you have to tune the flow rate through that sterilizer to accomplish the rate of sterilization required to either kill bacteria and algae or target parasites.
To target bacteria and algae, you are running a faster flow-through rate compared to targeting parasites. This is because the bacteria/algae and parasites don't require the same rate of sterilization. It also has to do with how fast the bacteria can replicate, by running through the sterilizer more often with a faster flow, you are ensuring the bacteria cannot reproduce faster than you can sterilize. Some bacteria can reproduce as fast as once every 20 minutes under the right conditions!
To target parasites, you will be running a much slower flow-through rate. This will achieve a longer contact time and a higher level of sterilization that will sufficiently disrupt the DNA of the protozoa. Furthermore, the reproductive cycles of said parasites are often measured in days rather than minutes or hours meaning they cannot reproduce faster than you can sterilize.
Since you cannot run both a fast and slow flow rate through the same UV sterilizer, you have to pick one or the other. The only way to achieve both is to run multiple UV sterilizers, each with a different flow rate which is a perfectly suitable approach if you are looking for ultimate sterilization.
UV Sterilizer Myths & Reef Tanks
We have already established that UV sterilizer can help prevent and control bacteria, algae, and parasites but won't eliminate them 100%. It's still important to have good husbandry practices alongside UV sterilization to achieve the best possible results. This leaves one myth on the table that used to be a common belief in reef aquarium circles.
It was once rumored that UV sterilizers will disrupt the natural food chain in your reef aquarium, killing phytoplankton and other beneficial microorganisms that feed your corals and invertebrates. It was stated that the result would have a negative effect on your tank inhabitants, which couldn't be farther from the truth.
While it is true that a UV sterilizer will kill live phytoplankton and other suspended organisms, these things get removed by your protein skimmer and other means of filtration anyway. Furthermore, we know that the natural food chain in your tank is nowhere near sufficient to sustain fish, invertebrates, and corals which is why we feed our aquariums. If you are feeding live phytoplankton to your tank, you simply turn off the UV sterilizer alongside your protein skimmer for a short amount of time. This allows the animals to gather the live suspended foods before the filtration can remove them.
Using a UV sterilizer will not starve your reef aquarium or disrupt the biological foundation. Beneficial bacteria primarily exist on surfaces meaning it is not suspended where the UV sterilizer can kill them. Furthermore, any sort of natural food chain in the tank is only supplemental to what we are manually feeding the tank inhabitants. This is exactly why live phytoplankton and rotifers must be propagated in a separate tank, then intermittently fed into your aquarium. These live foods won't be able to sustain a sufficient population in your display, with or without a UV sterilizer, and could potentially foul the aquarium water if left unremoved by your filtration.