CUV-236 36w UV Steriliser: What It Does for Green Water
By AllPetSolutions | Published: 2026-11-01
Category: Product Reviews
A 36w UV steriliser can clear green water, but only if the flow rate and contact time are right. Here's how to diagnose whether the CUV-236 is your fix.
Quick answer
A 36w UV steriliser will clear green water only when the pump flow through it stays within the unit's rated range and the bulb is working. For the AllPondSolutions CUV-236, that range is 6000–10000 L/H, so check your actual flow before buying.
Why flow rate decides whether a UV steriliser works
UV clarifiers rely on contact time: water must pass the bulb slowly enough for the UV dose to damage algae cells. If your pump pushes water through too fast, the unit becomes a pipe with a light in it. The CUV-236 is rated across a range rather than a single figure, which gives you room to run a larger pump without exceeding its maximum. That flexibility matters because most pond pumps lose flow as filters clog and pipe runs add head loss. A unit rated at one flow figure can be pushed past its limit by a pump that looks correctly sized on paper.
When green water is not a UV problem
UV sterilisers treat the symptom, not the cause. If your pond has high nutrients from overfeeding, decaying leaves or an undersized biofilter, green water will return as soon as the bulb ages. A UV unit is the right tool when the water is pea-soup green and clears in the bucket test, but it will not fix blanketweed, string algae or murky brown water. Those need different actions. Also check the bulb: UV output fades long before the bulb stops glowing, so a bulb that is visibly lit may still be too weak to clear algae.
Diagnose your green water before you buy
- Symptom: pea-soup green water that clears in a jar: Fill a clear jar with pond water and leave it in daylight for a few hours. If it clears, the green is free-floating algae and a UV clarifier is a sensible option. If the jar stays green or brown, the problem is suspended sediment or dissolved organics, not algae.
- Likely cause: flow outside the rated range: Measure your actual pump flow at the point where the UV would be installed, not the pump's headline rating. If it is above 10000 L/H, you need to split the flow, use a bypass, or choose a larger unit. If it is below 6000 L/H, the CUV-236 may be oversized and a smaller unit could be more efficient. A pump rated at 8000 L/H at zero head may deliver far less once filters and pipework are connected.
- Corrective action: install within range and maintain the bulb: Plumb the CUV-236 after the filter so it receives clear water, and keep the quartz sleeve clean. Replace the bulb at the manufacturer's recommended interval even if it still glows, because UV output declines with age. If green water returns within weeks of a new bulb, look at nutrient control and filter maintenance, not a bigger UV unit.
- Stop point: when to choose another option: If your flow cannot be brought within 6000–10000 L/H, or if the green water is caused by blanketweed or poor biofiltration, do not buy the CUV-236 as a fix. Address the flow or the nutrient source first. A UV steriliser is a clarifier, not a substitute for adequate filtration.
Product facts that matter for green water
- AllPondSolutions 36w Pond & Aquarium UV Steriliser CUV-236: Rated for a flow range of 6000–10000 L/H rather than a single figure. You can match it to a pump that sits above a typical single-figure rating without pushing the unit past its limit.
AllPondSolutions 36w Pond & Aquarium UV Steriliser CUV-236 - AllPondSolutions 36w Pond & Aquarium UV Steriliser CUV-236: A helix inside the case spins the water as it travels past the bulb. The spinning motion keeps water closer to the UV source for longer, protecting contact time even at higher flow.
