Goldfish Pond UV Sterilizers (Clarifiers)

UV sterilizers kill off algae that can't be filtered out.

Typical UV sterilizer used in goldfish ponds to remove green water.

Typical UV sterilizer (clarifier)

Goldfish pond UV clarifiers also called UV sterilizers, may be an easy option if your pond water keeps going green despite having a filter.

Clarifiers are low strength sterilizers that can only remove algae that cause green water. Sterilizers use a much stronger UV tube and much slower flow rate which can kill pathogens as well as algae. They are also usually much more expensive.

It doesn’t take much for pond water to turn green with an algal bloom. In a closed water ecosystem such as a pond, a seasonal change in sunlight hours or fish activity is enough to cause a bloom. Green pond water occurs most often during summer.

When we refer to an algal bloom, we mean suspended phytoplankton algae in the water table, not the filamentous types such as blanket weed that grow on pond walls plants and anything else they can find.
UV clarifiers are only effective against phytoplankton algae because it can pass through the clarifier.

Algae need two things to bloom, enough nutrients in the water and sunlight.

It is almost impossible to completely avoid an algal bloom unless the pond is frequently flushed or is naturally flushed by high rainfall that reduces the amount of dissolved minerals in the water to very low levels.

If your fish numbers are at recommended levels, and there are no other contaminants getting into the pond from either over-feeding or naturally, a UV clarifier may be an easy cure for green water.

UV clarifiers use a high intensity UV (ultraviolet) lamp that kills algae as it passes close to the lamp.

UV Clarifier Types

There are several UV clarifier options available:

Inline, fitted after the filter
Fitted internally in some filters which is very handy because you only have one power lead to deal with.
Fitted externally to some filters
Stand alone. This option allows the clarifier to be used when needed.

UV Clarifiers Must Match the Pond Water Volume

UV clarifiers, like filters, have to be the correct size for the pond they are being fitted to.

To work effectively, water flowing past the UV lamp must receive enough ultraviolet rays to kill the algae. If an inline clarifier is too small for your application, the water will pass through too quickly, not giving the UV time to kill the algae.

Buying a filter with a UV clarifier fitted usually guarantees the clarifier will be the correct size because the clarifier size is matched to the filter water throughput.

By size, we mean the wattage of the UV lamp in the clarifier. The higher the water flow through your clarifier, the higher the lamp wattage needs to be.

To remove algae that cause green water you need 1 watt per 40-50 gph (gallons per hour) of water flow.

Example:
Pond volume = 1000 gallons
Filter throughput required = 500 gph (pond volume once every two hours)
500/40 = 12.5
Therefore a clarifier with a lamp wattage of at least 12.5 watts is required for a pond of this size.

If you are buying a standalone clarifier, you don’t have to make allowances for a drop in efficiency from clogging or other variables that affect filters. All you need to do is buy the recommended clarifier for your pond’s water volume.

Install Goldfish Pond UV Clarifiers in Clean Water

Green water caused by an algal bloom

If you put a UV clarifier into a green pond, you will quickly clog the filter as the algae die and clump together inside the filter.

The same applies if you don’t have a filter and have only installed a UV clarifier. Clarifiers have basic coarse inlet filters that are designed to stop large objects from entering the body of the clarifier and impeding water flow. A clarifier isn’t meant to be used as a filter.

Goldfish pond after a UV sterilizer was installed.

Algae removed by UV clarifier

This is the same pond above about a week after a UV clarifier was installed.

The water wasn't crystal clear yet, but the algae bloom had been wiped out.

I had to keep cleaning the clarifier inlet filter pad regularly as it kept clogging up with dead algae.

Other Goldfish Pond UV Clarifier/Sterilizer Uses

UV clarifiers can be used for purposes other than controlling green water. The only problem is the water flow or gallons per hour must be much slower than is practicable for pond filtration.

Below is a list of possible uses for UV clarifiers and the approximate GPH flow rates:

Green algal bloom (clarification) - 40-50 gph (clumps the algae for filter removal, doesn't kill the algae)
Bacteria, some viruses (level 1 sterilization) - 20-30 gph
Parasites, resistant viruses (level 2 sterilization) - 5-10 gph.

UV clarifiers are only suitable for removing green algal blooms because of their high gph flow rates or low wattage UV lamps or both. UV sterilizers on the other hand, can be used for level 1 or 2 sterilization.

Buying and Fitting Goldfish Pond Clarifiers

If you already have a filter and want to add a clarifier, you now need to decide on the type that suits your pond.

For smaller ponds up to 400 gallons a standalone clarifier which includes a water pump is a good option. A water turnover of at least once every 2-3 hours is required for algae control so for a 400 gallon pond you will need a clarifier that has a 200 gph flow. The lamp wattage should be at least 5 watts. (200/40 = 5)

For bigger installations, an inline clarifier is a better option. Match the sterilizer to your filter's flow rate which should have a water turnover of at least once every 2 hours.

Amazon.com have a good selection under UV sterilizers and UV clarifiers.

Unfortunately, the claims made by some manufacturers are the same as for filters, and performance doesn't match the claims. Buy the well known brands or, if buying a cheaper model, halve the manufacturer's claims to ensure you get something that will work.

Expect a lifespan of 8000 hours or a year's continuous use out of your UV lamp. Over time the lamps degrade and become less effective.