Pond UV Clarifiers: Choosing, Fitting and Replacing the Lamp
Why pond water goes green, and what a UV does about it
A UV clarifier is there to deal with green water: the cloudy green water that makes it difficult to see your fish, even when the pump and filter are running. It treats the tiny algae floating through the unit so the filter can remove them. It does not replace the filter or clean the bottom of the pond.
We fit UVs as part of the pond’s filtration. The right unit can make a big difference to water clarity, but it helps to understand what you are trying to remove before comparing wattages and pond-size ratings. Start with the water itself, then look at the equipment.
Why is my pond water green?
Green water is usually caused by microscopic, single-celled algae suspended in the water. There may be so many of them that the whole pond looks like pea soup. You cannot pick out each cell by eye; you see the colour of millions of them together.
Algae need light and nutrients to grow. A sunny spell can make a noticeable difference, particularly as the weather warms up. Fish waste, uneaten food, decaying leaves and other organic matter add to the nutrients available in the pond. Soil and fertiliser washing in from the garden can add more. The RHS guide to pond algae explains how sunshine and nutrient build-up encourage it.
That does not mean you have to stop feeding the fish or shade every inch of water. It means checking the obvious sources: are portions being eaten, is waste leaving the filter, have leaves accumulated, and can runoff reach the pond? A UV helps control the suspended algae, while sensible pond care deals with what is feeding the problem.
New ponds can go green while the planting and filtration are becoming established. An established pond can turn green too, especially after a lamp loses effectiveness, a sleeve becomes dirty or the flow through the UV changes. “It was clear last year” is useful history, but it does not rule out a service problem now.
Green water is different from blanketweed
If the water itself is green and you cannot see far into it, suspended algae are likely to be involved. If you can see through the water but there are long strands attached to the walls, plants or waterfall, that is a different problem. A UV only treats water passing through its chamber; it cannot reach around the pond and remove attached growth.
Take a little water in a clear container and look at it away from the pond. This helps separate coloured water from green surfaces underneath it. Brown sediment, disturbed soil and other fine particles can also make a pond cloudy. Our pond algae guide explains the differences, and our blanketweed guide covers the attached strands.
How does a pond UV clear green water?
Inside a conventional enclosed UV clarifier, pond water passes a UV-C lamp protected by a quartz sleeve. The water and lamp stay separate. As the algae pass through, the UV-C damages their cells and interferes with their ability to reproduce. With the right exposure, this stops them multiplying unchecked and helps kill or disable the suspended algae.
The treated cells can clump together, making them easier for the filter to catch. The filter then needs to remove that material from the water, and the collected waste needs cleaning out. That last part matters: the UV has no waste basket of its own. The pond does not become clean simply because the lamp is on.
Think of it as two jobs working together: the UV treats the suspended algae; the filter collects the material that needs removing. Oase explains how UV clarification and filtration work together, while Evolution Aqua’s green-water advice stresses the need for effective mechanical filtration.
Water has to pass through at a suitable rate for the particular unit. Too much flow can leave insufficient exposure; too little flow through the whole treatment circuit means less pond water is being treated. That is why we check the UV, pump and pipework together, rather than choosing on lamp wattage alone.
A UV clears green water; it does not tell you whether the water is safe for koi. It does not deal with ammonia or nitrite, or remove the need to manage waste and feeding. Keep testing the water and looking after the biological filter, even when you can see right to the bottom. If the fish are unwell, use our koi health first checks and ammonia and nitrite advice.
Does every pond need a UV?
No. A planted wildlife pond is managed differently from a stocked koi pond, and a little natural growth is not automatically a fault. For a fish pond where suspended algae are spoiling the view, a correctly chosen UV is a useful part of the filtration. For blanketweed, sludge or unsafe water readings, the main work lies elsewhere.
If green water is the problem you are trying to solve, our pond UV clarifiers are a starting point. The next sections explain how to choose one that suits the pond and the filter you already have.
What size pond UV do I need?
Start with a realistic pond volume. If you do not know it, work it out before comparing equipment. A pond with shelves, steps or a sloping bottom holds less than a simple length-times-width-times-depth calculation suggests. Our pond volume guide helps you make a more useful estimate.
Then look at the type of pond the manufacturer has rated the UV for. A decorative pond without fish, a pond with a modest number of fish and a heavily stocked koi pond place different demands on the system. Manufacturers may give separate ratings, and the largest figure is often the one that looks most attractive in a short product listing.
Sunlight, fish numbers, feeding and the rest of the filtration matter too. A very sunny pond with large koi should not be treated as equivalent to a shaded ornamental pond just because both hold the same number of litres. Nor does a UV rating tell you how many fish the biological filter can support.
| Check this | What it tells you | What to avoid |
|---|---|---|
| Pond capacity | Whether the stated rating covers your kind of pond | Using the largest ornamental-pond figure for a koi pond |
| Treatment flow | How much water the unit is designed to treat effectively | Assuming a maximum-flow limit is always the preferred treatment flow |
| Minimum flow and pressure limit | Whether the planned pipework and pump suit the unit | Choosing on lamp wattage alone |
| Connections and installation type | How it fits the actual filter arrangement | Buying a filter-matched unit without checking compatibility |
| Lamp and servicing access | What you will need when the lamp or sleeve needs attention | Fitting the unit where the lamp cannot be withdrawn |
Read the koi rating, rather than just the biggest number
For example, the Oase Bitron Eco 120 W specification gives different maximum pond volumes: 120 m³ generally, 60 m³ with fish and 30 m³ with koi. That is 120,000, 60,000 and 30,000 litres respectively. Those figures describe this particular model; they are not interchangeable ratings for every Bitron.
The same specification also gives a 10,000 litres per hour minimum flow and a 50,000 litres per hour maximum. A unit can have a large pond rating and still be unsuitable for a small, slow-flowing installation. Check both sets of figures.
Why the flow through the UV matters
Think about the water actually passing through the clarifier. The number printed on the pump describes its performance under stated conditions; it does not tell you exactly what will arrive after the pipework, bends and changes in height.
If you are choosing a pump at the same time, use our pond pump sizing guide. It explains how to read the pump information without expecting the maximum figure to be what the finished installation delivers.
Some UV manufacturers give an optimum flow as well as a maximum. The Evolution Aqua evo55, for example, lists an optimum of 15,000 litres per hour and a maximum of 20,000 litres per hour. Use the optimum figure when comparing that model with your proposed flow. Do not choose a pump simply to push the maximum possible volume through the unit.
The evoUV Ti manual lists the same 15,000 litres an hour optimum and 20,000 litres an hour maximum for the evo55Ti. Choosing the Ti version does not mean increasing the flow beyond its instructions. Keep the manual for your particular version with the pond records.
Slowing the water down is not an unlimited solution either. If too little of the pond water reaches the treatment circuit, the system may not do the job you need. A bypass also changes how much passes through the UV. Match the flow through the whole circuit to the UV’s instructions. The pump and UV having similar maximum numbers does not make them a suitable pair.
If the plumbing has a bypass, keep a record of its intended setting and the reason for it. An unexplained valve position can make future fault-finding much harder. Never restrict a unit or alter its bypass without checking its minimum-flow, pressure and installation requirements.
Which type of UV fits your filter?
A separate inline UV clarifier
An inline unit has its own housing and sits in the water circuit. It can be a straightforward option where the filter does not already have a suitable UV, but it still needs the right connections, support and service space.
Our Evolution Aqua evoUV 55 W is an example of a separate unit. There is also an evoUV Ti 55 W in the range. Check the exact version when buying parts: a similar name or the same lamp wattage does not make every housing, sleeve or electrical component interchangeable.
A UV matched to a particular filter
Some installations use a UV designed to connect to a particular filter range. The connection is neater when the correct parts are used, and it avoids treating every component as a separate plumbing problem.
We fit ScreenMatic filters regularly, and our ScreenMatic guide shows several of our installations with different Bitron combinations. Match the Bitron and its fittings to your exact ScreenMatic model; they are not all interchangeable.
For larger ScreenMatic systems, compare the actual components in our ScreenMatic² 145,000 complete filter kit and the separate Bitron Eco 240 W. Check what is included in the chosen kit before ordering additional equipment.
UV modules on larger drum-filter systems
A large modular filter can use a different UV arrangement from a small garden-pond filter. The photograph below shows one of our Oase XL installations with Premium UV units on a swimming pond.
On a system like this, the pump arrangement, individual modules and service access all need considering together. Do not apply a small inline unit’s installation instructions to a different modular or immersed UV system. Clear water also does not establish that a swimming pond meets bathing-water requirements.
Where does a pond UV go?
The answer depends on the particular UV and filter system. For the evoUV range, Evolution Aqua describes the usual position after the circulation pump. In a pump-fed arrangement that normally puts it before the filter. With a gravity-fed filter, it normally sits after the return pump on the way back to the pond.
Those positions describe that manufacturer’s inline range. Filter-mounted Bitron units and UVs intended for installation inside a suitable chamber have their own instructions. Follow the exact manual rather than copying a diagram for a different product.
If you are changing the whole filter arrangement, our guides to Nexus filters and drum filters explain the distinction between pump-fed and gravity-fed systems. Sorting that out first makes the UV choice much easier.
Leave enough room to maintain it
Before fixing anything in place, check how the lamp and quartz sleeve come out. The unit might fit nicely between two walls but be impossible to service there. Think about access to the end caps, electrical connection and unions, as well as the length of the housing itself.
Put the model details somewhere you can read them later. A clear photograph of the rating plate, installation and connection sizes is useful when ordering a replacement lamp or asking for advice. Keep the manual with those photographs.
Never test an exposed UV-C lamp by looking at it. Use only the viewing indicator provided by the manufacturer on the assembled unit. UV-C can injure your eyes and skin. Isolate the equipment before servicing and follow the electrical and water-isolation instructions in the exact manual. Evolution Aqua’s UV safety advice explains why the lamp must remain enclosed.
Why is my pond still green when the UV light is on?
A glow tells you something is illuminated. It does not tell you that the lamp is still giving its intended UV-C output, that the sleeve is clean or that enough pond water is being treated.
Work through the simple checks before buying a larger unit. Make a note of what you find and change one relevant thing at a time, unless a fault needs immediate isolation. Replacing the lamp, changing the pump and moving several valves together makes it much harder to identify the cause.
| What you notice | Check first | What that means |
|---|---|---|
| The indicator glows, but the lamp age is unknown | The lamp record and exact manufacturer’s replacement advice | Visible light is not a measure of useful UV-C output |
| Performance gradually worsens | Quartz cleanliness, lamp age and water flow | Several service issues can look like an undersized unit |
| Water changed after pump or pipework alterations | Actual route through the UV, bypass position and model flow limits | The treatment circuit may have changed even though the UV has not |
| There is lots of suspended waste | Mechanical filtration, cleaning and circulation | The UV cannot remove the resulting debris from the pond by itself |
| There are long strands on the walls | Whether the problem is blanketweed | An attached algae problem needs different management |
| The fish seem distressed | Aeration, circulation and water tests promptly | Do not treat fish welfare as a cosmetic green-water problem |
Evolution Aqua’s green-water troubleshooting advice covers flow, lamp age and the need for effective mechanical filtration. That last point is easily missed: treating suspended algae and physically removing the resulting material are different jobs.
How quickly the water clears depends on the pond, how green it is and the equipment treating it. If the basic checks do not explain it, get the model details, pond volume and plumbing arrangement together before asking for help.
How often should you replace the UV lamp?
Use the instructions for the lamp and unit you actually own. “Change every UV bulb after the same number of hours” is too broad. There are different lamp types, control systems and manufacturer recommendations.
| Exact range or lamp | Manufacturer guidance | Keep this distinction |
|---|---|---|
| Evolution Aqua evoUV | Annual lamp replacement at the start of the season | Follow the correct lamp specification for your unit |
| Oase Eco 60 W replacement lamp, article 57077 | Up to 12,000 hours | Its compatibility list covers specific models; it is not every Oase UV lamp |
Keep a simple record of the fitting date and any lamp-hours reading. If the unit has a service counter, follow its instructions when replacing the lamp. Resetting a counter without doing the corresponding maintenance only makes the record misleading.
When ordering, have the full model name, article number and lamp details ready. The same wattage does not guarantee the same length, fitting or compatibility. Check the number of lamps the unit takes too; do not assume that the wattage in the unit’s name describes one individual lamp.
What is the quartz sleeve, and why clean it?
The quartz sleeve is the transparent barrier around the lamp that keeps the pond water separate from it. Dirt and scale on that barrier can get in the way of the UV doing its job. A new lamp inside a dirty sleeve leaves part of the problem untouched.
Evolution Aqua’s maintenance advice includes cleaning the sleeve during the season. Follow the cleaning method in your own manual, particularly where seals, cleaning mechanisms or approved descaling products are concerned. A sleeve is fragile: forcing it, scratching it or overtightening a fitting can turn a routine service into a leak.
Make sure all replacement seals and components are the correct ones. After reassembly, carry out the specified leak checks before restoring the lamp to service. The evoUV manual, for example, specifies a 24-hour water leak test before the lamp is installed. Check your exact model’s sequence rather than improvising.
Should the UV run at night and through winter?
Follow the manufacturer’s operating advice. Evolution Aqua recommends continuous operation for evoUV when treating green water. The fact that the sun has gone down is not, by itself, a reason to switch that unit off overnight.
Some equipment is designed to control lamp operation automatically. The Bitron Eco range is an example. An automatic control programme provided by the manufacturer is different from manually switching an ordinary UV on and off because the water happens to look clear.
Winter decisions also depend on frost protection and whether the equipment remains in service. If a unit is taken out of use, follow its isolation, draining and storage instructions. A housing with trapped water can still be at risk even though the lamp is switched off. Do not assume that equipment described as weatherproof can be submerged or buried.
If you use a pond treatment, check that treatment’s label and the equipment instructions for any UV-off period. Products differ. Write down when the UV should be restored so it is not accidentally left off afterwards.
How much does a pond UV cost to run?
You need the whole appliance’s electrical consumption, its operating hours and your own electricity price. Lamp wattage alone is not always the same as the complete unit’s draw.
An example using a hypothetical 60 W appliance and a 30p/kWh tariff:
60 ÷ 1,000 × 24 = 1.44 kWh per day.
1.44 × £0.30 = £0.432 per day, or £12.96 over 30 days.
The tariff and appliance draw here are illustrative. Use your unit’s whole electrical consumption and your own tariff for a useful comparison.
For an automatically controlled unit, running hours can differ from the hours it remains connected to power. Check the manufacturer’s information and, where suitable, measured consumption. Include replacement lamps in your buying comparison as well as the initial price.
Before you order, have these details ready
- The pond’s approximate volume, fish stocking and sun exposure.
- The exact filter and pump models, plus whether the filter is pump-fed or gravity-fed.
- The planned flow through the UV and any manufacturer minimum, optimum and maximum figures.
- The connection sizes, available mounting space and lamp-removal clearance.
- The full model and part numbers if you are replacing an existing lamp, sleeve or unit.
For advice on a current installation, clear photographs of the complete circuit and rating plate are more useful than a close-up of an unidentified glowing indicator. Send us those details and we can help choose a UV clarifier that suits the rest of the equipment.
Other pond UV questions
Will a bigger UV fix poor filtration?
No. A UV clarifier and biological filter do different jobs. If the filter is not managing the fish waste, investigate that directly. Start with ammonia and nitrite testing and the filtration arrangement.
Can I use a UV on a wildlife pond?
First decide whether the pond needs that equipment at all. A planted wildlife pond is managed differently from a heavily stocked koi pond. Do not choose a UV just to remove every sign of natural growth; identify the issue and the purpose of the pond first.
Do I still need aeration?
A UV clarifier does not replace aeration. Choose oxygenation and circulation for the needs of the fish and filter; our pond aeration guide explains air pumps, diffusers and larger systems.
Should I turn off the UV when using a pond treatment?
Read the exact treatment label before adding it. Evolution Aqua explains that UV can break down medication in the water, so follow the treatment manufacturer’s directions for switching the UV off and back on. Do not assume one product’s instructions apply to another.
Does a UV keep fish free from disease?
No. It only acts on material that passes through the unit and receives the required exposure. It does not treat a disease already on the fish or replace good pond care. Evolution Aqua makes that distinction in its advice. If fish look unwell, use our koi health first-checks guide and get appropriate professional help.
Should I replace the UV if it has stopped working?
Not automatically. Record the exact unit, fault indication and service history, then check the manufacturer’s troubleshooting advice or ask for help. Isolate leaking or electrically suspect equipment. A lamp, sleeve, seal or electrical fault needs identifying properly before buying parts.
