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UV filtration in your fish tank or small fish pond Answered

Year after year the topic "I have a fish tank" seems to go more out of control.
What was once a hobby just to have some fish can now be a design feature both in your home and inside the tank.
Realistic looking lasdscapes, optical illusions that make you think the tank is much bigger and the list goes on.

But one thing that now always pops up is the must have thing of UV filtration.
Or to be precise: UV-C sterilisation!
Now, if we trust Wiki and our big water suppliers then UV-C will literally kill anything alive that comes into contact with.
So of course it would be a good thing to have for your tank - or not?

UV-C is very dangerous for your eyesight and quite harmful for your skin!
Looking into a proper UV-C lamp without protection means you can go blind!
Even good sunglasses might not have enough protection in the UV-C range, so only use them for additional protection but never without and glass between you and the lamp!
Don't be a fool! Treat UV-C seriously!
You would not look into the full sun with your sunglasses and would not expose your eyes or skin to a powerful laser, UV-C is to be treated the same way!

Let's start by using some boring text to explain the concept a little bit.
On a large scale special and quite powerful systems are used to treat our drinking or pool water.
Here special UV-C lights with a wavelenghts of 260nm or below are used to shine through the water passing by.
There are two key factors here.
a) the wavelenght
b) the water flow rate and the corresponding time the water is in contact with the UV light
To ensure all bacteria, viruses, algae and other harmful organics are dead the water must circulate for long enough so even the last water molecule had a few seconds of exposure.
All this only works good with "crystal clear" water for obvious reasons as otherwise the UV has to be even more powerful to pass through.
Single cell organisms literally crack into pieces similar to being exposed to gamma rays, more complex cells like algae have their cell membranes damages and the DNA suffers as well causing reproduction loss and early death.
Even some chemicals break down, most importantly here chlorine based substances.

Differences within the UV-C range!
If you bothered to check Wiki about the topic of UV-C you will already know that only certain wavelengths within this spectrum will actuall be powerful enough to do what we want it to do.
And here is the first problem for us hobby users.
Most cheaply advertised "sterlisation lamps" you find in places like Ebay are actually totally useless.
Stating to be selling a UV-C light to sterilze your water in such a case is still not considered to be fraud though.
Simply because it still does what it supposed to do, just very slow and with very little effect.
Only the so called "short wave" UV-C range is powerful enough!
To avoid loosing business during the times of the biggest hype in 20 years no seller will actuall state the available wavelengths.
That means without this info anywhere you can be certain the advertised lamp is of little to no use.
Even those advertised to be short wave UV-C might not be the real deal.
However, if a decent manufacturer is behind the actual lamp used it is possible to check the datasheet for these performance figures - but again most cheap systems come with no-name lamps inside.
Check the prices for a reputable UV-C light with the same lamp fitting, e.g. G23 and you will see it might cost more than your entire system.

Ok, you have a poper short wave UV-C lamp or consider getting a canister filter with one in it....
Never, ever test your lamp without proper protection!!!!!
UV-C will damage your eye within seconds!

If you system or lamp does not provide a viewing port or shine through area then you have to place a piece of glass between you and the light!
UV-C won't be able to penetrate normal window glass but will pass through quartz glass.
Place the lamp in a box and cover with the glass.

How make proper use of UV-C sterilisation...
The replacement lights are quite expensive, so let's see how to get the most out of them.
As said before exposure is the key factor so the flow rate of the UV system must match tank size and flow rate of your filter system.
Canister filters with a build in lamp should be designed to match but I will tell you later what to look for ;)
Most of us will prefer to have a in-line system if there is already a good canister filter at work, so I will focus on those and rop in solutions.
If you compare in-line system you might notice that some quite small and low power units claim to allow for the same flow rates as for example 40W units.
Some are fraud and just want to sell while others use simple physics to make the claim true.
A good system will utilise an auger like "ramp" that forces the water to circulate around the tube many times - causing up to ten times longer exposure rates.
Others create this sprial effect more like a vortex with some diverters and modified inlets.
The later seems to be less efficient though with low power lamps.
An in-line system should be on the outlet side of your canister filter so the best quality water will pass through it.
A drop in solution should be used alone and without the existing normal filter pump you might have in there.

Ok, got it, but how do I actually use it now?
Despite common thinking a UV-C system should not run 24/7 like your normal filter.
You really only need it to solve problems you should not have in a healthy tank!
It is not a magical solution to make your underlaying problems go away ;)
Let's start with the most common reason someone buys a UV-C system: An algae or bacterial outbreak causing greenish or milky water.
If that developed slowly over a period of weeks then you would be better off to do a good clean of the tank and filter plus a decent water exchange.
A few drops of meds will do the rest.
And if you constantly get algae growing on your glass, ornaments and plants then your nutrient levels and water quality is not right anyway and needs a good check.
But of course there is also the problem of light - too much for too long and unwanted gree appears everywhere.
If in doubt reduce the light power, shade out natural light or reduce the on time for your lights.
Having said that we now face the problem of a sudden outbreak after introducing new fish or plants.
If you don't have a quarantaine tank chances are that sooner or later you get unwanted or even harmful guest into your tank.
Here the UV-C will be beneficial, which is why a canister filter with build in light should have a seperate switch or power supply for the light.
After an outbreak or while introducing new life into your tank the UV-C will remove a lot of the things that we don't want to bring along.
For new life I leave the light on non stop for a week, that is for a small 4ft tank with 200 liters.
To control an outbreak it depends on how bad it is.
I assume here you can still see the back of your tank  but that the water either appears greenish or slightly milky from bacteria.
As a personal thing I prefer to to remove and clean my filter material before treating a severe outbreak.
Once done I fill the filter with a mix of activate carbon material and fine filter wool.
Reason for this quite simple:
The outbreak causing stuff is already in your filter material and will be a constant source of re-infection.
And since breaking down all this bad stuff causes even more bad stuff to be produced as biological waste we want to discard it properly once done.
Using just fine filter wool and activated carbon also reduces the flow rate bit if compacted ;)
Now we can turn on the light and pump and forget about it for a while.
It is not recommended to run UV lights on a timer as you want them on all time to prevent short lifespan and have ongoing treatment of the water.
Good idea to take a picture at the same of a day from now on to compare and check results.
After 3 days the water should definately be clearer, if not then either your filter material is packed too losse or the lamp is no good.
Once the water appears to be clear do a readin test - take a newspaper behind the tank and check if the text is clear - blurry means the water is still not clean.
You will reach a point where the water quality will not further improve as much as in the days before.
This is the time where you discard or clean out to dry your filter material and put the original stuff back in.
The activated carbon should be discarded of course.
You cleaned filter material will now need a certain time to grow enough good bacteria to go back to the old performance.
During this time you should still leave the light on.
In most cases with enough fish and plants in the tank a week should be sufficient.
After that you can leave the light off and keep the tank fit and healthy.

Special case: Algae everywhere!
Especially after getting a new plant you can end up with quite pesty algae growth.
Be it these long ghost hair types or in a bad case the black stuff growing on plants, ornaments and the glass.
I have even seen tanks with algae covering the entire bottom of the tank causing the gravel to look like carpet.
Here I can only advise to set up a quarantaine tank for your fish.
Then remove all infested material for manual removal and cleaning.
Infested plants should be cut clean and what can be boiled should be boiled in water for a few minutes.
Now start scrubbing in the tank with ongoing water replacements.
I prefer to let everything settle over night without any bubbler or pump running.
This way I can suck up a lot of sediment the next day.
If you can remove all plants and fish you can now use hydrogen peroxide and add it to your tank water.
But this is only feasable for small desktop tanks.
Before using the UV as above to cure an outbreak you should consider all water one last time.
Allow at least 2 weeks with ongoing water checks before adding plants back in and another week before placing your fish back in the tank.
The week before adding fish should be used to monitor the plats for any signs of algae you might have missed - if you find any remove it!
A week after the fish is back in you can turn off the UV light.

Underwater UV-C light!?
In most online stores you will find quite cheap UV lights to be advertised as underwater or in tank use.
Although it might sound tempting you should be well aware of the dangers of using them.
The glass of your tank will block the harmfull UV rays but the water surface won't, so either don't ever look at it or use proper sunglasses with real UV protection.
Apart from the dangers to you these lamps are not just cheap in price but also cheaply produced.
That means there is no way of telling how much or how little UV-C is produced.
If they are good then you still need to know in what type of tank setup you can use them.
As plants can tolerate a bit of UV a placement as far away from the nearest plant should do, especially if you can place a bubble wall betwenn light and plants.
The fish is another thing as some seem to be unaware of the danger in their tank.
This means they can get too close to the light but I have not found any articles explaining how harmful UV-C is to fish or their eyesight.
I guess once your fish starts to bounce into everything you know... ;)
My advise is to stay away from the idea of hanging a UV-C lamp in your tank, the risk for you and your tank is just not justified.
If you need to go cheap then get two or thre of these lamps so you have spares.
But use them externally ;)
Meaning: Take a UV proof plastic container of small size and place the light in there.
To be really safe tape the lid and all holes for the hoses with black tape.
Place the container above the water level of your tank and if you only have an internal filter pump push a suitable sized hose into the outlet to feed into you canister.
Check how high you pump can make it and place the outlet or overflow slightly below this level.

When to change the light?
If you made it all the way down here then you might already had the benefit of using light to "cure" your tank.
Now we are faced with the high replacement cost for the lamp itself.
Ususally only flouroscent tubes are used.
It is always good to check after purchase what type of lamp and manufacturer (if there is one) was used.
In some cases the system itself is like an inkjet printer: Just a cheap way to make you buy the consumables.
Let's say you new in-line filter was priced at $100 to have a nice round number, some are cheaper some much more expensive.
The lamp used might be an exotic type and not even be available easy, so before you buy your system check where you can get spares, not just the lamp of course.
A replacement lamp can be as ceap as 20 bucks or cost even more than your system if you need to order it elsewhere.
The quartz glass sleeve can break too meaning you then need a lamp and cylinder.
Going with a reputable brand and paying a bit more certainly helps to get spares in the future.
Let's just assume you either got your system in bulk due to the price of replacement lamps or can get them at a reasonable price.
UV-C lamps are not like your normal flouroscent light tubes you have around or maybe even on top of your tank.
Consider them like the tubes used in the now unhealthy tanning beds.
After a certain amount of time they no longer produce enough of the short wave UV light that we need.
As you can't see it and most of us won't have the means to specifically measure it we have to trust manufacturers recommendations.
For most good brands the numbers are the same: 8000 hours max.
Considering the costs it does make sense to keep written track of the usage.
Not too hard since we won't use them like normal lights but instead have them on for a week or more without turning them off.
I recommend to have a replacement at hand long before you need it.
A lamp can fail premature, crack or simply burn out.
The 8000 hours are based on 24 hour usage, so one day on, one day off.
This could mean for us the lifetime can be slightly longer but I would not go over 9000 hours.
As a rule of thumb: If the water does not show good signs of getting clear on day thre the lamp is due.


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Jack A Lopez
Jack A Lopez

3 years ago

I forgot to mention, for anyone using Wikipedia to learn more about germicidal uses for ultraviolet radiation, the knowledge is not all in one article. There is a handful of, what I think, are relevant articles, here:


Also there exist LEDs that can produce UV-C light. Although these are currently kind of new and rare, in the next 10 years these may become the dominant devices for producing this kind of light. I mean, this is what we can expect if LEDs take over the UV-C germicidal niche, in the same way they made obsolete, incandescent and fluorescent visible lighting devices, over the last 10 years, previous to the present.


Reply 3 years ago

Only problem is the correct wavelenght for the said application.
For example those lamps used in butcher shops are usually of lower power and in the upper region of the UV-C band - to protect the people working in the light.
The lamps used on a professional base to kill harmful bacteria and such by surface radion are mostly at the lowest wavelength possible for one side of the spectrum and go up to medium lenght UV-C.

LED's are certainly taking over sooner rather than later as it is already indicated by the price drops for higher wattage UV-C lamps.
On a professsional level it just makes sense to invest a few hundred bucks more on LED system to avoid replacing the the flouros every 9 to 12 months.

The one thing I miss so far on all UV-C lights I had so far is safety warnings for the user.
Even for a "desinfection lamp" sold here in special cleaning supply stores there is no warning in regards to eye and skin protection.
It is just assumed the user would never look directly into the light or be subjected to strong reflections from a shiny surface.

Jack A Lopez
Jack A Lopez

3 years ago

Once again, I thank you for this unsolicited outpouring of your brain, on a particular obscure, but interesting, topic.

This has caused me to wonder: Is there an easy, low cost, low-tech method for to detect the presence of UV-C?

I mean, I know there are electronic sensors, like photodiode with fused silica window, designed to detect just that part of the spectrum. But is there a low-tech way to do this?

Also I want a method that is less complicated than setting up an aquarium filled with fish, and other organisms. I know that is the way you test a lamp, for to see if it is producing UV-C; i.e.

"After 3 days the water should definately be clearer, if not then either
your filter material is packed too losse or the lamp is no good."

But I want a method that is easier, faster, and cheaper.

Maybe there is a chemical method. Do you know if there are any simple, easy to observe, chemical reactions facilitated by a good UV-C lamp?

Does the air surrounding the lamp smell like ozone?

Will the invisible radiation from this lamp bleach common dyes? Will it bleach, make white, colored pH indicator paper, or maybe ordinary paper stained with beet juice, or some other common food coloring?

Will it make certain materials fluoresce, or luminesce?

Also I guess I want a test that can differentiate between UV-C, and the longer, less energetic, and less sexy and dangerous, wavelengths, like UV-B, UV-A, and violet and blue.

Anyway, this is sort of a question I have been pondering.

In the even I come up with an easy answer for this myself, I will write back to this topic and let you know about it.

By the way, I have just two possible sources of UV-C in my possession at the present time. One is a gizmo called "Dry and Store(r)". That is actually a brand name. The device itself is this tiny cabinet like thing with a oddly funereal appearance, like it might be a coffin, or a mausoleum, about the size needed for a dead gerbil or goldfish.

However, the actual intended use of this gizmo is as a tiny cabinet that can dry and ostensibly disinfect, a hearing aid.

Thankfully I don't have any hearing impairment myself, but I found this , "Dry and Store" gizmo used, cheap, at a thrift store, and I bought it mostly because, at the time, I had no clue what kind of appliance it might be, and basically I bought it just to take apart for to discover what it had inside.

That's kind of a long story, but what the D&S appears to have inside is, a fan, a desiccant brick, and a clear, fluorescent lamp, with driver circuit for the same. Also there is a switch to sense when the drawer is closed, and the drawer being closed is a necessary condition for the circuits that activate the fluorescent lamp.

So I figure that's kind of a good sign; i.e. the designers of this gizmo have taken steps to make sure light turns on, only when the door is closed. I mean, this design suggests the light is actually dangerous to look at, and as you've already mentioned, this is characteristic of a "good" UV-C lamp.

The other possible source of UV-C I have, is my arc welder. It only makes flickering, intermittent, broad spectrum light, but I dunno. There might be some UV-C in that light.

Josehf Murchison
Josehf Murchison

3 years ago

OK do you expect someone to read or remember every thing you wrote?

Now you didn't say fresh water or salt water tank.

The best way to keep your tank clean is Biodiversity.

Plants to feed on the nitrates and other chemicals in the water as well as they oxygenate the water.

Filter feeders that clean the water like, shell fish and fish that feed algal like mussels, clams, big head carp.

Bottom feeders that clean the glass and gravel like, snails, crayfish, and catfish.

Last something that eats all the above to prevent overpopulation.

Even if you use UV you need a filter to remove the solids and the filter feeders remove the solids.

UV only kills it doesn't remove the food source for bacteria.


Reply 3 years ago

Well I wrote it down so noone needs to remember it all.
And I did not specify fresh or salt water as makes no difference in this case.
If you check again you will notice i mentioned to use just filter wool and activated carbon during a long UV treatment for curing an outbreak.
Just so most of the really bad stuff is out when putting the original filter material back - really helps a lot as you then still have good bacteria in the filter to get going.
And yes, I also mentioned that you don't need a UV system for a fit and healthy tank ;)

I mainly wrote all this so people who consider getting a UV system due to the hype can see some facts first and get the chance to avoid ever needing one :)

Josehf Murchison
Josehf Murchison

Reply 3 years ago

I never liked UV systems, we had them for purifying drinking water but as fast as they were installed in homes a short time later they were removed even faster.


Reply 3 years ago

Drinking water is a whole different story, especially on a residential level.
Proper multi stage filtration does far more these days than what most people think.
If you need more water quality than what comes out of your tap the best option is to check with the supplier first.
And of course to compare the quality on a tap close to the meter to rule out problems with the plumbing in the house.
Where I am we get the water from three different storage dams, depending on water levels subsidised with "water" from a desalination plant.
That means the taste and sometimes the color is slightly different then what you got used to.
Many people here use water filter systems at least for the kitchen taps but they also love to totally neglect them.
Had cases where they complained the water pressure is down.
Turned out the filters were totally blocked and already growing algae inside....

Josehf Murchison
Josehf Murchison

Reply 3 years ago

OK here is funny for you; yesterday we got our first snow, this morning the water mane broke. So once the township fixes the water mane we are in for brown water for a week. Thank good for Brita Water filters.

This never happen when I lived on the farm, we got our water from an artesian well.


3 years ago

I calibrated a spectra scope using a quartz mercury arc fluorescent light but knew my glass glasses blocked the dangerous UV-C... HA ha, , , the lenses were plastic ! or poly-carbonate and sun burned my eyes real bad..

Spent a horrible night with sand in my eyes, that was what it felt like, and next day rode my bike to a doctors office zigzagging from shadow to shadow..

Point is, be careful and shield pump reflow through the quartz water tube in a light proof box where the UV lamp resides away from everything but germs...