Aquarium UV sterilizer: the radical fix for green water
Against persistent green water, the UV sterilizer is the most effective weapon: the water clears in 3 to 7 days. Expect 30 to 60 € for an aquarium model, 60 to 150 € for a pond. But let’s be honest: UV treats the symptom. Without fixing the excess light and nutrients, the green water will return as soon as you switch it off.
How does a UV sterilizer work?
A UV-C lamp sealed inside an opaque tube irradiates the water flowing past it, destroying the suspended single-celled algae responsible for green water, along with some free-floating bacteria and parasites. Plumbed into the filter outlet or fitted with its own pump, it only treats what circulates: algae attached to the glass and decor are unaffected.
What UV wattage for what volume?
- 5 W: up to 150 litres of aquarium, 25 to 40 €.
- 9 to 11 W: 150 to 400 litres, 35 to 60 €.
- 18 to 36 W: ponds up to 10,000 litres or more, 60 to 150 €, often built into pressurized filters, as mentioned in our guide to the garden pond.
- Matching flow rate: the water must pass the lamp slowly; too strong a flow halves the effectiveness.
Is UV dangerous for the tank?
Not for fish, shrimp or plants: the UV only acts inside the unit. It doesn’t destroy the good bacteria in your filter, which are attached to the media rather than floating free. Two precautions all the same: never look at the lit lamp (eye burns), and switch the UV off during medication or when seeding with starter bacteria, which it would neutralize.
How do you stop green water from coming back?
Green water is always the sign of an imbalance: too much light (direct sun, lighting beyond 8-10 hours — see our article on lighting duration and algae), too much food, not enough plants. Fix those three points and the UV will only be needed for the occasional few-week course. Remember to replace the bulb every year (10 to 20 €): it still glows, but it no longer sterilizes.
Frequently asked questions
Should the UV run permanently?
In an aquarium, no: a 2 to 4 week course is usually enough. In a pond in full sun, it runs continuously from April to October.
Does UV get rid of white spot (ich)?
It reduces the free-swimming parasite stage and limits the spread, but doesn’t cure infected fish: treatment is still needed.
My water is cloudy and white — will UV help?
Not much: white cloudiness is a new-tank bacterial bloom that resolves on its own. UV specifically pays off against green.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.