UV (for U and ME)
The Questions
I don’t wish to make this a long sticky post, but. . .you know me by now.
I’ve been asked more than a few times about the use of an ultraviolet (UV) unit in marine aquarium systems. “Should I use one?” “Does it kill diseases?” “Will it cure diseases?” “Will it kill Marine Ich?” “Is it worth having one?” and similar questions. Hopefully I can provide enough information for you to make your own decision.
What Is It?
The UV unit is sometimes referred to as a ‘sterilizer’ a ‘UV filter’ and other common names assigned by hobbyists. Actually, it doesn’t sterilize. Actually, it doesn’t remove anything from the water, like a mechanical filter would. It does however alter the contents of the water. I refer to it as a “UV unit” or just “unit.”
The UV unit consists of a light of high energy (ultraviolet bulb) shining on marine water passing by. There is the bulb outside of a ‘sleeve’ (often made of quartz glass). Inside this sleeve the water passes by the bulb. The whole unit is encased so the ultraviolet light is not seen by the hobbyist or people.
In this post, I’ll refer mostly to the energy that the bulb makes as ‘energy.’ It is however a type of radiation that is dangerous to humans, too. It can affect eyesight fairly easily, not to mention skin.
When a small enough lifeform or free molecule moves by the light, and is close enough to the light, and is exposed long enough to the light (proper flow rate), the light alters the lifeform or molecule. It can disrupt proteins (like DNA and RNA) so that the organism can’t replicate; it can alter chemistries inside the organism so that it can’t ‘eat;’ it can affect cell walls so that the organism can’t properly function; it can alter proteins and chemicals ‘dissolved’ in the water. In the case of living organisms, the applied energy may lead to the death of the organism or rendering it unable to reproduce.
Notice the condition by which the UV unit works properly: proper sizing of light (the quantity of energy needed); the flow rate of the water and organisms/chemicals going past the light; how close the light is to the organisms/chemicals; and the size of the organism that can be so affected. In a more abbreviated form, what is important is how much energy is given the organism/chemical and whether or not that organism/chemical can be affected by that amount of energy.
I’ll mention here that the proper flow and energy applied by the bulb is what makes the unit effective at its job. Putting the wrong bulb (supplying the wrong energy) in the unit or using a too-fast water flow is like doing nothing.
The Uses and Effects
The device is used for two primary purposes. One is the control of micro algae and the other is the control of more difficult micro organisms (amoeba).
In ponds and other large bodies of water, it is used to control waterborne algae. It can turn ‘green water’ to clear water. Since micro algae is an organism that is easily affected by the UV energy, the unit doesn’t require that much energy. So the water flow can be faster and/or the energy of the light can be lower.
The device also (though not one of its primary uses) disrupts proteins that are in the water column. This can alter the proteins, including breaking them apart, to make them more readily available to organisms for eating or using. The breakdown of such proteins can make them easier to remove by the protein skimmer and carbon filter. For this affect, the energy must be high and the flow slow for the optimal exposure to the light.
Another primary use is for the unit to kill kill microbes in the water. Unfortunately, the microbe must be small enough for the energy to affect it AND unfortunately, not all the water in the system will be passing through the UV unit and therefore, not all organisms will be killed. Like when the UV unit is used to control micro algae – it doesn’t kill it all it just kills what goes through the unit. For this affect, the energy must be high and the flow slow for the optimal exposure to the light.
The concept of the unit being a sterilizer is a mis-perception. It doesn’t affect all lifeforms AND it doesn’t get to affect the ones that don’t go through the unit. Lastly, not all of the same microbes are affected. That is to say that even when the bulb wattage (energy) and flow is proper, the energy only affects a certain percentage of the organisms/chemicals that pass by the bulb.
In this way, the UV unit is like a protein skimmer. The skimmer is working and removing proteins, but is the water free of proteins? No. Of course not. Not all water goes through the skimmer, does it. Does the protein skimmer remove all proteins from the water that goes through it? Of course not. It merely reduces the quantity. Like proteins, the lifeforms are continually being bred in the display tank and other places in the system and not all go through the unit AND when they do pass through the unit, not all will be equally affected. Do you see how much 'unlike a sterilizer' is the UV?
It should now be clear that the unit doesn’t sterilize, but the unit is good at controlling certain microbial lifeforms. It prevents the microbes from ‘blooming’ to a nuisance level. Do not mix the concept of ‘controlling’ with ‘elimination.’ The unit does not eliminate anything from the water in the system.
It certainly kills some microbes, including the free swimming stage of Marine Ich, but these pathogens stay near the bottom of the aquarium, sticking near live rock and substrate. Few actually find their way to the UV unit. So all that the UV unit can accomplish is to keep the population of these pathogens from getting out of hand. That is the control.
The Value
So then what is the value of the unit? It is valuable when the marine system is overstocked with fishes and/or overfed. That is to say, the water contains an abundance of nutrients (e.g., proteins), bacteria, and/or micro algae.
The value of the unit is its constant use. By constantly treating the water it can effect a reduction in the amount of waterborne organisms and alter chemicals.
Larger marine lifeforms like pods can often get through a properly designed UV unit without being affected enough to kill or prevent reproduction. Some are affected. Certainly, the UV unit cannot kill all marine lifeforms that pass by the energy source (bulb). Size does matter. The smaller the lifeform the most likely it is adversely affected by the exposure to the bulb’s energy.
I would include a UV unit in a marine system that is overstocked with fishes or which is being overfed. (Both of these conditions encourages excessive proteins and bacteria in the water and also will help prevent any bloom of the bacteria). I would also definitely use a UV unit when there is less than optimal water clarity. Beyond these conditions, the use is optional. However, the downsides have to be considered.
The Downsides
The UV unit puts energy into the water (as noted above). The water gets a lot of heat from this exposure to the bulb. If the UV unit has a dedicated pump (which it should if the system pump moves water too fast), that pump adds heat to the water. With the extra heat, the marine system water may run too hot for fishes and/or invertebrates. This will require fans, extra evaporation, a chiller, or other means to lower the marine system water temperature on a continual basis.
If the water runs too warm, the fish metabolism goes up and the fish grow faster or may not even tolerate such heat – that is die before their time.
If evaporation is used, then the hobbyist will find that much more add-back water is needed and may be required daily. If a chiller is needed, the electric bill can take a leap up!
Running the UV unit costs money and not just a little. The bulb will need regular replacement (usually every 4 to 6 months). The pump will require maintenance. The UV unit will need to be kept clean so that all the bulb energy goes through the sleeve and reaches things in the water. (The sleeve should be inspected and removed and cleaned when needed). Some units require replacement of o-rings and gaskets on a regular basis.
The Incorrect Uses
Some hobbyist imagine they get good results from either running the wrong size unit (too small for the flow, or the flow too fast for the unit), or only turning the unit on during a part of the day. If this is really your thought, don’t invest in one. They can only do their job when they run all the time. Remember that even when running 24/7/365 they only process part of the water. Using them any less just means even less water is being processed.
Using a unit with an outdated bulb or which hasn’t received regular maintenance and cleaning is a waste of money. If the hobbyist is not going to perform their end of the project, then no sense in getting a unit OR keeping one running.
The unit will not kill nor prevent disease. If it were that easy, it would be a boon to the trade. You still need to quarantine new marine life before putting into the system to verify health and having been trained to eat. The UV unit doesn’t remove any pathogen (bacteria, protozoa, etc.) from the entire aquarium water – only the water that passes by the bulb.
Ultimately, the really sad part of the UV unit is the lack of standardization among the many manufacturers. The rating of the bulb is critical to the amount of energy the bulb puts out from the first day to the last day. You see, with every passing day, the bulb puts out less and less energy. Hence the need for bulb exchange. But does the manufacturer rate the unit for when the bulb is new, 1 month old, 2 months old, or when the bulb is at the end of its recommended life (e.g., 6 months old)? The later is a better rating system. When the bulb needs to be changed the hobbyists wants to be assured that the bulb was still doing its job, but close to dropping below its rating. Some less-than-reliable manufacturers sell less expensive units when what they are doing is shorting the life of the bulb – guaranteeing what the light will kill but only when the bulb is new or a month old. What is needed is a standard system whereby the energy of the bulb at 4 and at 6 months is known to still be doing its job. That is to say, the purchase of one can be challenging and if the correctly sized one is not obtained, it will be a waste of time and money.
The Plan
Make your decision to use a UV unit from what it actually does and how it actually affects the marine system water, NOT from myths and ‘myth-information.’ I hope the above will help you decide.
Don’t be fooled. The UV unit will not make up for poor water quality; poor system maintenance; or for not using a proper marine life quarantine process.
Add your questions to this thread on UV units, if you have them!
Last edited:
