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What’s the difference between a UV clarifier and a UV sterilizer?

I’ve noticed people get confused when these two terms are mentioned. Let’s clear up the confusion.

A uv clarifier and a uv sterilizer are not actually two different ‘types’ of filters. On the contrary, these two words refer to the same type of filter, and there’s no actual difference in the units themselves.

The terms “clarifier” and “sterilizer” are just descriptions of the possible function of the unit. You can use a unit as a clarifier or as a sterillizer. It depends on what you want for your pond.

If your intention is to just clear up the pea green color in your pond, without necessarily having a super sterile environment, then a uv clarifier is enough for your purposes. A clarifier exposes the water to enough uv to kill free floating algae. This is the most popular option for ponds.

On the other hand, if you wanted to take it one step further and wanted to control pathogens like viruses, bacteria, and protozoa in your pond as well, you need to use a UV light as a sterilizer. Sterilizers expose the water to more UV than a clarifier. This can be helpful for example to prevent spread of disease from one fish to another through the water.

(Note: obviously the uv sterilizer only affects organisms floating through the uv tube, so any infestations that are already ON the fish, or bacterial diseases fish might have, will not be affected.)


A unit turns out to be a clarifier or a sterilizer depending on 2 factors:

-> the wattage (W) of the uv bulb
-> and the flow rate through the uv light tubing


generally, uv filters that people call ‘sterilizers’ are of a higher wattage (40 W, 60W) compared to ‘clarifiers’ (which can be 5W, 9W, 18W). But this is only a very rough guideline.

For example, a 25W sterilizer in a small sized pond (say 1000 gallons) , will only be able to act as a clarifier in a bigger pond (for example 4000 gallons).

But even these figures aren’t very reliable, because a lot depends on other factors, such as:

  • the amount of fish in your pond
  • the amount of sunlight the pond receives
  • and as well on the manufacturers specifications of the unit (the size and length of the chamber around the bulb matters),
  • on the age of the lamp,
  • the cleanliness of the lamp (amount of calcium deposits)
  • and the temperature of the water…

Flow rate:

Simply put, if the water moves slowly along the bulb it sterilizes, and if it passes more rapidly then it clarifies.

The longer the organisms are irradiated with UV light, the more chance that the cell wall will be destroyed (=sterilized). But at the higher flow rates the DNA is merely mutated and the cells cannot reproduce (=clarified).

A unit of 10 watt can be a sterilizer too if the water flows through slowly enough. (But in this case, you’d have to have a small pond (say 500 gallons) for the filter to keep up with algae reproduction.)

Similarly, a 200 watt unit would act as a clarifier if the flow rate is high enough.


It depends on the situation

So if you’re wondering whether to choose a uv clarifier or a sterilizer, it comes down to the specifics of your actual situation. For example: koi ponds (with more fish and few plants) will require uv units two to four times larger than well planted fish ponds.

Here’s what you have to take in account:

  • What is the size of your pond? Obviously a bigger pond will need a bigger sized uv system with a higher UV bulb wattage.
  • Do you have a lot of plants in it? If you have a lot of plants, then these will assist in the removal of nutrients (leaving less food on the table for algae to grow).
  • Do you have a lot of fish in your pond? (More fish means more nutrients for the algae.)
  • Does your pond receive lots of sun, or is it more shaded? If it gets a lot of sun and warmth, the algae will grow much quicker. In this case, a sterilizer would be recommended.
  • Whether you already have a pump that you also intend to use for the UV light. Your choice of UV light depends on the flow rate of the existing pump.

Directions are usually given on the models themselves. Or, for clear directions on what type of UV pond filter to choose, see our “pond uv filter buying guide”.