Goldfish pond UV sterilizers, also called UV clarifiers, 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.
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, not the filamentous types such as blanket weed that grow on pond walls plants and anything else they can find.
UV sterilizers are only effective against phytoplankton algae.
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 sterilizer may be an easy cure for green water.
UV sterilizers use a high intensity UV (ultraviolet) lamp that kills algae as it passes close to the lamp.
There are several UV sterilizer options available:
UV sterilizers, like filters, have to be the correct size for the pond they are being fitted to.
To work effectively, water flowing past the UV sterilizer lamp must receive enough ultraviolet rays to kill the algae. If an inline sterilizer 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 sterilizer fitted usually guarantees the sterilizer will be the correct size because the sterilizer size is matched to the filter water throughput.
By size, we mean the wattage of the UV lamp in the sterilizer. The higher the water flow through your sterilizer, 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.
Pond volume = 1000 gallons
Filter throughput required = 500 gph (pond volume once every two hours)
500/40 = 12.5
Therefore a sterilizer with a lamp wattage of at least 12.5 watts is required for a pond of this size.
If you are buying a standalone sterilizer, 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 sterilizer for your pond’s water volume.
If you put a UV sterilizer 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 sterilizer. Sterilizers have basic filters that are designed to stop large objects from entering the body of the sterilizer and impeding water flow. A sterilizer isn’t meant to be used instead of a filter.
This is the same pond above about a week after a UV sterilizer was installed.
The water wasn't crystal clear yet, but the algae bloom had been wiped out.
I had to keep cleaning the sterilizer filter pad regularly as it kept clogging up with dead algae.
UV sterilizers 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 sterilizers and the approximate GPH flow rates:
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.
If you already have a filter and want to add a sterilizer, you now need to decide on the type that suits your pond.
For smaller ponds up to 400 gallons a standalone sterilizer 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 sterilizer that has a 200 gph flow. The lamp wattage should be at least 5 watts. (200/40 = 5)
For bigger installations, an inline sterilizer 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.
And the same rule for pH levels applies to ponds as it does aquariums; clean, clear water doesn't automatically mean the pH is correct.
Keep checking the pH regularly, especially when rainfall is low.