UV Sterilisers : Your Weapon in the Battle Against Algae

Whether you are the owner of a home aquarium or pond, you will be aware of the constant battle to prevent algae growth. In many instances, the presence of algae will be inevitable, regardless of how well or how regularly your fish tank or pond is cleaned.

But where does it come from? An algal bloom can occur in saltwater or freshwater sources. The ecological imbalance is caused by the presence of too much organic material from dead leaves, excess feed or fish waste. If the amount of organic material is too high, the water loses the ability to maintain adequate oxygen levels. The absence of oxygen and the abundance of nutrients provided by organic material allow algae to thrive

Apart from looking unsightly, the deoxygenation of water caused by the algae will have a detrimental effect on your fish and aquatic animals. 

What Causes Algae to Grow?

Put simply, like any plant, algae needs only three basic requirements in order to grow; light, water and nutrients. Whilst all three will be present in all aquarium and pond environments, fluctuations in any of these can cause algae to flourish.

There are a lot of changes during the transition into the warmer summer months – the increase in temperatures, sunlight, and available nutrients from decaying plant matter and fish waste - all of which collectively promote algal growth.

  • Light Changes – If your pond is set under a lot of direct sunlight and little shade is provided, this is a common cause of algae. Equally, fish tank owners can reduce excess algae by putting aquarium lights on a timer, as an extended light cycle can promote an abundance of algae.

  • Water Changes – Algae can be made worse by using mains water to top-up ponds, as the chlorine affects the natural balance of the water. Instead, fill your pond with siphoned off rainwater, or use a dechlorinator and water conditioner, to ensure the water is safe for your fish and maintain your pond’s biological balance.

  • Nutrient Changes – for pond keepers, an excess of nutrients can be caused by a sludge build up at the bottom of the pond, fertilisers filtering into the pond, and common pond debris such as leaves, weeds and fish faeces, providing a feeding ground for algae.

How Does a UV Steriliser Work?

As many fish keepers would agree, one of the best ways of tackling single-celled algae, which causes ‘green water’, is by adding an Ultra Violet Steriliser to your filtration system.

In short, a UV steriliser works by penetrating and breaking down the algae cell wall, allowing them to adhere to each other. This causes the algae to flocculate, or clump together, enabling the pond filter to remove it from the water.  

The UV bulb inside the steriliser is attached to a quartz sleeve, which protects the bulb from the water, and the electrical components of the light are enclosed within a ballast and housing unit. In your pond or aquarium, the water is pushed slowly through the steriliser’s quartz sleeve by a pump, where high-intensity UV light passes through the water. This sterilises the bacteria and microorganisms present in the water by breaking their cell membranes or damaging their DNA or other organelles in the cells.

Some own-brand AllPondSolutions UV Sterilisers have a unique design which feeds the water through a special helix, meaning the water does not just run through the UV but spins inside; giving more exposure to the UV and providing greater efficiency before the water exits the outlet.

In addition to destroying bacteria, ultra-violet sterilisers can help reduce the spread of free-floating algae, however, whilst it will kill algal blooms, it will not actually eradicate them. A UV steriliser is not a replacement for good biological and mechanical filter, which is still necessary to physically remove algae from the water.

Choosing the Right UV Steriliser for Your Fish Tank or Pond

It is important when selecting a UV steriliser that you choose a unit specifically for the flow rate of your pump and size of your pond or aquarium; otherwise there will not be enough water turnover to kill off the unwanted bacteria and algae.

You will also need to consider which type of UV steriliser is right for you; a hang-on, or an in-line or in-tank UV:

Internal UV Steriliser

Hangs on to outside of your tank – this is a smaller steriliser, with a lower wattage.  May require an additional pump installed. Easy to install and budget-friendly, these can be ideal for low maintenance fish tanks.

Suitable for smaller aquariums.

In-Line or External UV Steriliser

As the name suggests, these sterilisers are plumbed into the aquarium or pond filtration system. These have larger UV bulbs and water capacity, giving them increased efficiency.

Suitable for larger aquariums and ponds.


As an aquarium owner, before purchasing a UV steriliser there are a few considerations that you may like to make. When keeping a freshwater aquarium, often a heavily planted tank will keep algae and bacterial issues at bay by competing directly for the light and nutrient sources that they need to survive. If you keep rift valley cichlids, you may also wish to encourage the growth of algae, as this is a suitable food source for mbuna and other African cichlids.

It is also important to note that a UV steriliser will not be able to differentiate the good bacteria from the harmful bacteria in your aquarium. If you are considering a steriliser for a marine fish tank, you may wish to put your steriliser on a timer to function during the day, as free-floating beneficial plankton usually hides away in rocks during daylight hours.

Maintenance and Aftercare for your UV Steriliser

The effectiveness of your UV filter will not only depend on selecting the most appropriate model for your tank or pond but on regular maintenance to ensure that it is working as efficiently as possible.

Here are two easy ways to make sure your UV steriliser is in top working order:

  1. The quartz sleeve – ensure that this is checked monthly and cleaned with rubbing alcohol to remove residual waste. Take care when cleaning the sleeve as it can become easily scratched, which as a result would make the UV steriliser less effective. If it is not kept clean, insufficient levels of UV light will be able to infiltrate the water and the algae will not be killed. Some larger UV sterilizers are fitted with in-built wiper mechanism for easier maintenance.

  2. The bulb – this should be replaced every 6 months for an aquarium and 12 months for a pond UV to guarantee the light penetrating the water is strong enough to kill the maximum amount of bacteria and algae.  It is also essential to replace O-ring seals or keep them lubricated when changing the bulb to ensure the unit remains watertight. Take care when fitting or changing a bulb not to look directly at it - when the unit is switched on, this can cause permanent damage to your eyes.

NOTE – For safety reasons, always disconnect the unit from the electricity when performing maintenance to protect against a possible shock.