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How to use UV light in aquarium to control Bacteria and Algae

Is perennial algae blooms and constant cloudy water in your aquarium? Are bacterial and other parasitic infections plaguing your pet fish ? Well, it just may be time to find out how to use UV light in the aquarium to sort the problem.

UV sterilizer will help clean your aquarium water and stem the nagging microorganisms spread. Subsequently, you’ll be able to put infections spread under control effectively.

How to use UV light in aquarium

However, it is absolutely not a substitute to standard filtration system (both biological and mechanical) to keep the tank clean. Strictly treat it as an “insurance policy” backing up your proper aquarium care.

How to use UV light in aquarium:

Before discussing how UV light in the aquarium works, let’s first define and understand; what UV light is and what is a UV sterilizer.

What is UV light?

For starters, the sun emits short-wavelength light. This light splits into 3; UV-A, B and C. It is only UV-C, the shortest of the 3 that contains germicidal effect.

This germicidal effect in UV-C is able to break molecular bonds and alter electronic configurations. Through that, it is effective in sterilizing.

What is a UV sterilizer?

The basic definition is that “it is a ultra-violet light tube which kills single cell organisms passed through it by denaturing its DNA”.

Or simply put,

“A filtration device that emits UV light able to mutate a microorganism’s DNA when passed through/near the bulb”.

Its effect is halting the growth and reproduction of the exposed micro-organisms. Subsequently, besides killing them, their life cycle is also shortened.

Worth noting is that, UV light is cancerous. However, if used properly, it is harmless to life in the fish tank; whether fish, corals or even other invertebrates.

So what does a UV sterilizer do?

UV sterilizer stops and/or controls spread of infections through water from one fish, coral and/or invertebrates to another.

It however is only useful on free-floating one celled organisms. It is especially useful for algae control in ponds.

To be effective, the target microorganisms MUST be in the water flowing to the UV sterilizer.

UV light in the aquarium however has no residual effect and won’t eliminate organisms already attached. In short, it’s ineffective on those attached to fish, substrate, plants or corals. 

How does a UV sterilizer work?

UV sterilizer uses a germicidal fluorescent lamp to emit light of about 253.7 nanometers wavelength. Exposing the parasite infested water to the wavelength’s light running over the bulb in the UV system, gets it sterilized.

Irradiating of microorganisms happens as the light penetrates through the bacteria/algae infested water. Subsequently mutation of their DNA takes place. As a result therefore, their growth or multiplication is halted.

This is because nucleic acids will be destroyed. This in effect disrupts the microorganisms’ DNA disabling performance of their critical cellular activities.

The altering of the simple weak chemical molecular bonds to stronger ones is by the UV light’s high energy photons. It is a result of conversion to easy to break down simple celled DNA material. This in a nutshell is how UV light in aquarium works.

Benefits of UV light in aquarium:

From the above read, you by now can tell that a UV sterilizer will help improve your aquarium’s health. It is effective in killing bacteria, parasites, viruses and even algae. In-fact, they’re effective in preventing algal outbreaks.

I bet you don’t want your fish to suffer stress from infections and diseases or die from them all together. Well then, an aquarium UV sterilizer will kill the disease causing pathogens.

An algae bloom can be devastating. In a short span, algae can use-up all the oxygen in the tank. The result is severe stress from fish suffocation. Subsequent stress greatly compromises the immunity of fish.

You need a UV sterilizer for algae control to quickly kill the algae. This will restore the healthy water conditions.

Finally, it will spare you resources (time, money, energy et al) to salvage a disease outbreak condition. UV light in the aquarium reduces chances of disease causing pathogens and microorganisms buildup.

How to install UV light in aquarium:

Having laid out the basics, the next step after learning how to use UV light in aquarium is how to install it. Good thing is, just like installing common canister filters, it’s simple.

However, you must strictly follow the manual.

Well, to acquaint yourself, dismantling it for study and familiarization is recommended.

Caution is however highly advised because of its fragility, especially the sleeve.

Where it’s easily reachable and away from inlets obstruction is where to put UV light in aquarium.

It’s important to note that, it’s the last equipment to place in the aquarium.

Biological, mechanical and/or chemical filters should all precede it.

Moreover, if it’s the type to attach to the filter, it should equally be placed last in the filtration line.

The goal is to have water first pass through the filter before the UV sterilizer. That way, waste particles/solids are first removed. As a result, UV light will be impactful.

UV sterilizer performance influencing factors:

Are UV sterilizers worth it? Well, the worth of an aquarium UV light sterilizer is judged based on its performance. This is however highly influenced by the following factors.

The UV light penetration:

Light penetration is vital for microorganisms’ DNA mutation. Therefore, inability of water to allow UV light’s penetration makes it ineffective.

Water turbidity inhibits light penetration. In effect, bacteria and algae aren’t reached. Therefore, biological and mechanical filters are essential to first clear the water before it runs through the UV sterilizer.

In addition, unlike freshwater, brackish water has low light penetration.

Further, if a dirt film builds on the bulb or sleeve, light will be obscured hence lowering its penetration. Cleaning up the bulb will improve the situation.

Finally, the bulb’s distance from the water matters too.

The size of the organism:

Unlike small sized organisms that require less UV light to inert, lager ones like protozoa will require higher amounts.

In addition, some bacteria strains have higher resistance to UV light. As a result, they may require higher amounts.

Now, you may be asking, does UV light kill beneficial bacteria in aquarium? Well, UV light will not kill already attached bacteria. It’s for that reason that it can’t save already infected fish. In that case, you have to use medication.

During tank cycling therefore, when the bacteria are building, you should switch it off. This is because the bacteria are yet to attach to the substrate and/or bio-media in the aquarium.

Therefore, after it’s attached, the short and precise answer then is, NO.

Similarly, to the question “is a UV lamp dangerous for fish?”, the answer is NO.

Power, temperature and age of the bulb:

Different organisms need varied power level to inert. The bulb’s power is measured in watts. The more the watts, the higher the UV light it will generate.

Low temperatures lower the yield. The ideal temperatures range to yield effective UV light is 104-110°F. Therefore, to sustain high UV output, you’ll need to insulate the bulb from aquarium’s water cooling effect. Therefore, your bulb will need a quartz sleeve for protection.

With time however, the bulb’s ability to generate UV light declines. Subsequently, radiation is highly reduced.

It is for that reason that you should replace it every 6 months or so.

Contact time of UV light:

This refers to exposure of water to the light. It is also referred to as the dwell time.

The flow rate determines the waters exposure to the light. The longer the exposure, the more effective the light is.

You may also increase the contact time by having a longer bulb.

Slowing the flow rate increases contact time. Subsequently, more microorganisms are killed.

The question I assume you’d be having now is; how long to leave UV light on in aquarium? You can have it on for as long as is practically possible. However, you can occasionally turn it off for the sake of aquarium plants that will do with the algae. In addition, if you have algae eater fish, you may consider it too.

Turn over time and the Size of the tank;

Turn over time is basically the time needed to have the whole aquarium’s waters sterilized. It’s important to match the tank with the right UV sterilizer. So, before getting one, first accurately answer the question; what size of UV sterilizer do i need?

A big tank, served by a small UV sterilizer will only increase the turn over time. The sterilized water is soon infested with the microorganisms if it’s taking long to serve the whole tank. It’s essentially ineffective.

How to pick the right UV Sterilizer:

Like most other things, there are multiple choices of aquarium UV light sterilizers to pick from. But before you do, ensure it’s the right fit for your tank.

Important factors to consider before picking one are;

  • The pump and the bulb’s power,
  • Microorganisms that you intend to kill (remember some like Protozoa need higher light amount),
  • And obviously the size of the aquarium

Any shortcomings with an aquarium UV light sterilizer?

By now, I hope feel like you sufficiently know how to use UV light in aquarium. However, you should also know of a few of the inherent limitations of aquarium sterilizer;

  1. UV aquarium light is NO substitute to biological and mechanical tank filtration. You must clean your fish tank as often as is necessary. (Read on how often to clean your aquarium).
  2. It is only effective on free-swimming microorganism. It’s unhelpful on already attached organisms, be it on fish, coral or even substrate.
  3. While running, the device may interfere with tank parameters. For instance, mismatching aquarium sterilizer with the tank size may cause unnecessary warming up of the tank waters.
  4. The sterilizer is incompatible with medications. UV light will denature the medicines formulations. As a result, the broken down elements are pollutants. Now, if fish is already ailing from the microorganisms’ infections, during medication, switch it off.


Using UV light in aquarium will no doubt help to keep the tank clean and healthy. It will help keep in check the development of bacteria and other parasites.

Having it will be great and make your tank healthier and safer.

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