Two of the biggest problems any aquarium owner will face are fish disease and nuisance algae. Anything that helps to reduce or prevent both of those is a welcome addition to the aquarium cupboard, and an aquarium UV steriliser can do just that.

UV stands for ultraviolet light, and a special UV bulb emits light at 254 nanometres. At that wavelength, the light is damaging to our eyes (so it’s safely concealed inside the UV body,) and damaging to small life forms including bacteria, viruses, algae, and parasites.

By passing aquarium water near to the UV bulb, those organisms can be damaged or killed by the UV rays, resulting in aquarium water with reduced levels of free-floating algae and disease pathogens.

What you need to fit a UV steriliser to your tank 

Aquarium UV sterilisers consist of a UV bulb, ballast, and a power supply. To get them working you either need to buy an internal filter with a built-in UV, or use a powerhead, pump or external filter to connect to the unit’s hosetails, and pump aquarium water through the unit.

An external power filter is ideal, as it will also pre-filter the water of physical debris, enabling better penetration of the UV light. A UV can be placed inline, either on the inlet or outlet tube of an external canister filter and hidden away in the aquarium cabinet.

Choosing an aquarium ultraviolet steriliser

Most aquarium UV sterilisers have either an aquarium volume, a maximum recommended flow rate, or both, stated on the packaging. Contact time is absolutely key to effective sterilization, so the slower the water flow through the unit, the more time the pathogens spend exposed to harmful UV, and the more effective it will be. 

You can’t have too much UV, but you can have too much water flow through the UV, so select a unit that will cope with the flow rate from your pump or filter, or turn the filter flow down. For large systems or powerful pumps, multiple UV lights can be fitted inline to increase the contact time and UV sterilisation capacity.

A bypass or even a small, separate pump can be used in order to ensure that flow rates are nice and slow through the unit. 

Changing the aquarium UV bulb

UV lamp performance drops off rapidly, so ideally they should be changed every six to 12 months. Handle UV bulbs carefully so as not to break them. They sit inside another glass tube called a quartz, which needs to be clean and free of limescale in order to let the UV light penetrate through the quartz, and affect the water.

Clean or replace the quartz at the same time as you change the UV bulb, and check the O rings and seals too. Use gloves to avoid getting greasy fingerprints on the bulb or quartz.

FAQs on aquarium UVs

Will a UV steriliser prevent Whitespot?

It can help to. The Whitespot parasite is actually quite large when compared to viruses and bacteria, so needs maximum contact time. Fit a large UV steriliser to your tank and slow the flow through it right down.

Whitespot parasites have several life stages including one that dwells in the gravel, so keep quarantine and hospital tanks substrate free, and in particularly severe cases, consider removing the substrate and other decor entirely from the main tank.

A UV is an effective weapon against some disease pathogens, but don’t think your aquarium will be free of all diseases. Standard disease treatments may still need to be used, along with a rise in temperature, in the case of Whitespot in tropical fish.

Are fish that have been kept in UV treated systems more susceptible to disease?

No, it’s not true. Many fish will be exposed to UV on their way from fish farms to aquariums and it only helps in lowering disease pathogens that flow directly past the UV tube.

UV has no effect on fish with parasites on their skin, and no free-swimming lifecycle stage. Nor does it negatively affect general fish health in any way.

Can a UV steriliser be used on any type of aquarium?

Yes, fresh or saltwater, coldwater, tropical or marine, goldfish to guppies to Discus and clownfish, UV can be used on all types of aquarium and will benefit water clarity and fish health. 

What happens if my filter is more powerful than the maximum flow rate recommendation of the UV?

It will be less effective at tackling algae and pathogens, so consider adding another, second UV sterilizer, or turning the flow on the filter down. 

Can I save money and use a pond UVC?

You can, but in general, the void space is much closer in a steriliser than it is in a clarifier.

The bulb is the same, and the pond unit will state that it is suitable for much larger water volumes, but that's clarifying, not sterilizing, so disease pathogens may be less affected.  

A pond UVC may also not be suitable for use with saltwater.