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UV-C Clarifier

Water clarifiers for aquaristics

UV-C clarifiers are also called UV-C sterilisers or water clarifiers. These technical devices are used in freshwater and seawater aquaristics, but are also used as useful accessories in garden ponds. In this article we explain exactly what a UV-C clarifier is and what it can do.

How the UV-C clarifier works

A UV-C clarifier cleans and clarifies the passing water from the aquarium or garden pond by irradiating it with UV light. For this purpose the so-called UV-C burner, which consists of a special UV-C radiating LED or fluorescent lamp, is installed inside the housing of the device. "Light" is perhaps a somewhat superficial term here, because UV-C light is not visible to the human eye. The term refers to radiation in a certain wavelength range. In general, UV stands for ultraviolet radiation, whereby the UV range in the wavelength spectrum is subdivided into UV-A, UV-B and UV-C radiation. The UV-C range covers wavelengths from 100 to 280 nm. A water clarifier also operates in this range. Even though these sterilizers are often referred to as UV clarifiers in aquaristics, this always refers to the UV-C range, which is characterised by very short wavelengths and high-energy radiation. This has a disinfecting effect by inactivating microorganisms and stopping their propagation. The radiation damages the DNA of the unicellular organisms.

JBL - ProCristal UV-C-Klärer

As a rule, a UV-C clarifier has two connections for this purpose, with which the device is connected to a pump or an external filter and the aquarium via suitable hoses. The pump or filter ensures that the water from the aquarium can flow through the UV-C steriliser and past the UV-C lamp installed in it. Note: This process is more effective if the water flow is lower and therefore slower! With a slower flow, the microorganisms and unicellular algae contained in the water are exposed to the radiation for longer and can therefore be deactivated better. If the UV-C clarifier is just connected to an existing external filter, it would be a good measure to slightly reduce the flow rate. In addition, the UV-C water clarifier should be installed in the filter circuit after the external filter so that the water cleaned by the filter materials (retention of dirt and suspended particles) can then pass through the UV-C clarifier.

What problems does a UV-C clarifier help with?

First and foremost, a water clarifier ensures very clean, low-germ and clear water through UV-C irradiation. By deactivating potentially harmful microorganisms, the bacterial load in the aquarium water is reduced and the water quality improved. This process can prevent diseases, because fewer germs in the water mean less risk of infection or spreading of fish- or shrimp diseases. UV-C clarifiers can also be used as support in specific cases such as Ichthyo (white spot disease), as they also kill the free spores of the pathogens in the water.
The use of a UV-C steriliser is recommended for sensitive fish or invertebrate species which require a low bacterial load in the aquarium water, as well as for the breeding of young fish.

Floating algae

A UV-C clarifier also prevents the growth of algae in the aquarium, as unicellular algae and algae spores are also prevented from spreading by the neutralising effect of the high-energy radiation. In concrete cases of milky turbidity, a bacterial bloom or an infestation with green floating algae, a UV-C-Sterilizer reliably removes the problems in a very short time.

By the way: A UV-C steriliser has no notable negative effects on the "good" bacteria in the aquarium. The pollutant-degrading filter bacteria that are involved in the nitrogen cycle in the aquarium, such as nitrobacter or nitrosomonas are mainly bound in biofilms on all surfaces in the aquariums and less in the open water. In this respect, there are no concerns to be had.

UV-C clarifiers and plant aquariums

In aquascaping and plant aquaristics, UV-C clarifiers are somewhat in disrepute because they allegedly "destroy" the liquid fertilizers in the water. This is not quite true, because these water clarifiers have no appreciable influence on the nutrient content in the aquarium water. They destroy however by their irradiation the chelate complexes of certain fertilizer components. These chelate complexes, such as ETDA, are very often used in so-called complete iron fertilizers in order to keep trace element nutrients such as iron available for the plants in the water over a longer period of time. This effect is eliminated by the use of a UV-C clarifier. This device therefore only has a negative effect on the aquarium or aquascape if you use an iron fertiliser chelated in this way, for example as a weekly fertiliser (or for an even longer period of time). With a daily fertilization the effect can be disregarded and there shouldn't be any problems.

When should a UV-C clarifier not be used?

A UV-C clarifier may be inappropriate in certain situations due to its functionality. As described in the chapter above, a combination of a weekly fertiliser with an artificially chelated iron fertiliser and a water clarifier is counterproductive, as the steriliser cancels out the long-term effect of the fertiliser. This can be remedied by switching to a daily fertiliser.

The simultaneous operation of a UV-C sterilizer is also not always compatible with medication treatment in the aquarium. Here it is necessary to follow the instructions of the manufacturer of the medicine used.

Bacterial products

If bacterial preparations (usually in liquid form) are added to the aquarium, a UV-C clarifier can also have a counterproductive effect. In principle, it cannot do any harm to deactivate the water clarifier for all cases for some time when such a bacterial agent is added. If required by the instructions for use of the bacterial additive used, the UV-C water clarifier should always be switched off.


A water steriliser based on UV-C is a great aid in aquaristics that ensures clean and pure water. Even if one refrains from a permanent employment, it can be nevertheless absolutely worthwhile to have such a device ready - in such a way one is prepared against sudden algae emergence or cases of illness.