UV dose: why flow rate matters more than watt rating
Walk into any aquarium shop and UV sterilisers are sold by wattage. But wattage is just the power of the lamp — it tells you nothing about whether the water passing through actually receives a useful dose. Flow rate is the variable that matters, and most hobbyist units are run far too fast to deliver it.
The science of UV inactivation is built around one central number: dose, measured in millijoules per square centimetre (mJ/cm²). Every target organism — algae, bacteria, protozoan cysts, viruses — has a known dose requirement for a given level of inactivation. Get below that number and UV barely touches it. Exceed it and you achieve reliable kill rates. Wattage determines the intensity of the lamp; dose is what the water actually receives.
What UV dose actually means
UV dose — also called fluence — is the product of two things: the intensity of the UV light passing through the water, and the amount of time the water spends in the UV chamber. Written as a simple relationship:
Dose (mJ/cm²) = UV intensity × exposure time
UV intensity is fixed by the lamp — a 9W low-pressure UV lamp emits a characteristic amount of 254 nm radiation. You cannot easily change it. But exposure time is entirely controlled by your flow rate: the slower water moves through the UV chamber, the longer each litre spends in the UV field, and the higher the dose it receives.
This means flow rate is the most powerful variable in the whole system. A flow rate that is twice as fast delivers half the dose. A flow rate that is half as fast delivers twice the dose. The lamp's wattage is the same in both cases.
Why manufacturers' "recommended" flow rates are misleading
UV steriliser boxes typically list a maximum recommended flow rate — often expressed as litres per hour. These figures are almost always based on the dose required to clear green water or achieve a basic bacterial reduction, which requires only 20–30 mJ/cm². If your goal is protozoan inactivation or reducing the load of harder-to-kill pathogens, you need a higher dose — which means running the unit at substantially lower flow.
How much dose do different targets actually need?
The peer-reviewed literature — particularly the comprehensive review by Hijnen et al. (2006) in Water Research — provides dose requirements for a wide range of organisms. Translated into aquarium terms:
| Target | Dose for 1-log (90%) | Dose for 3-log (99.9%) |
|---|---|---|
| Green water algae (Chlorella) | ~10 mJ/cm² | ~30 mJ/cm² |
| Cryptosporidium oocysts | <7 mJ/cm² | <20 mJ/cm² |
| Giardia cysts | <7 mJ/cm² | <20 mJ/cm² |
| Typical aquarium bacteria (E. coli equivalent) | 3–9 mJ/cm² | 20–30 mJ/cm² |
| Ich theronts (free-swimming) | ~10–15 mJ/cm² (estimated) | 30–50 mJ/cm² |
| Adenovirus | >50 mJ/cm² | >150 mJ/cm² |
The table shows why UV works so well for green water but gives mixed results for general disease prevention. Clearing green water only requires 20–30 mJ/cm² — achievable by most units at moderate flow. Reliably inactivating bacteria and protozoan parasites requires 20–50 mJ/cm² — achievable only at lower flow rates. And viruses like Adenovirus are practically outside the range of any hobbyist UV unit.
How to think about your unit's actual dose
You cannot directly measure the dose your UV unit delivers without specialist equipment. But you can reason about it from the basic relationship between flow rate and exposure time.
Most hobbyist in-line UV units (the type plumbed into your filter return) have a UV chamber volume of roughly 0.2–0.5 litres. At a flow rate of 500 L/h, water spends about 1.4–3.6 seconds in the chamber. At 200 L/h, that rises to 3.6–9 seconds. A lamp that delivers a given intensity has roughly 2.5 times more opportunity to irradiate each litre at the lower flow rate.
A practical rule of thumb that follows from the science:
- For green water only: the manufacturer's recommended maximum flow is probably fine.
- For reducing bacterial pathogens: run at 50–60% of the recommended maximum flow.
- For protozoan control (Ich, Velvet): run at 30–50% of the recommended maximum flow, accepting reduced turnover rate.
- For reliable virus inactivation: not achievable with typical hobbyist equipment regardless of flow rate.
"Halving the flow rate doubles the dose. This single adjustment can be the difference between a UV unit that barely touches pathogens and one that reliably inactivates them."
The turbidity problem — green water defeats itself
UV radiation at 254 nm is attenuated by anything dissolved or suspended in water. Tannins, dissolved organic compounds, suspended particles, and — crucially — the algae cells of a green water bloom all absorb or scatter UV light before it reaches other organisms.
This creates a self-limiting problem: the denser a green water bloom becomes, the harder it is for UV to penetrate far enough to inactivate all the cells. The cells near the lamp are inactivated quickly; the cells at the far side of the chamber may receive a much lower dose. A severe green water bloom will clear more slowly than a mild one, not because the UV is less effective against the algae, but because the algae itself is blocking the UV from reaching everything.
The same attenuation applies in tanks with high tannin content (heavily botanicals-dosed setups), very high dissolved organic loads, or any form of cloudy water. UV is most effective in clear, low-tannin water.
Your lamp's output is falling
All UV lamps — both low-pressure (the most common type in hobbyist units) and medium-pressure lamps — degrade in output over time. The UV-C output of a standard low-pressure mercury lamp typically falls to about 65% of its initial value after 8,000–9,000 hours of operation. That is roughly 11–12 months of continuous use.
A lamp running at 65% output delivers 65% of the dose it did when new — all else being equal. A unit that was marginal at the right flow rate when the lamp was new may deliver a meaningfully sub-threshold dose by the end of its first year. Most manufacturers recommend annual lamp replacement regardless of whether the lamp is still producing visible light (it will be — lamps lose UV output long before they stop producing visible glow).
If you are using a UV unit specifically to manage a recurring problem (green water, elevated pathogen risk), note when the lamp was installed and replace it at the 12-month mark.
A practical sizing approach
Rather than matching UV wattage to tank volume (the common but misleading advice), think about it this way:
- Decide what you're trying to achieve — green water only, or general pathogen reduction?
- Choose a flow rate that delivers the dose you need — this will be lower than the manufacturer maximum for anything beyond green water.
- Check the turnover rate that results — your tank still needs adequate overall flow for gas exchange and biological filtration. See the guide to flow in a planted tank for what adequate coverage actually looks like. If running the UV at a low enough flow rate reduces overall tank turnover too much, either run a separate UV pump circuit at low flow (independent of your main filter return), or accept that UV is best suited to targeted use cases (clearing a bloom, managing an outbreak) rather than continuous 24/7 operation.
- Replace the lamp annually regardless of visible output.
Green water: the one clear win
If your only goal is clearing green water, a correctly sized UV unit running at manufacturer-recommended flow will almost certainly work. Green water algae is one of the most UV-susceptible targets. Even a modestly powered unit will clear a bloom within a few days. What UV will not do is prevent the bloom from returning if the underlying cause — excess light, unbalanced nutrients — is not addressed.
For the science of what UV actually inactivates and why some things pass through unaffected, see the companion article on UV sterilisers: what they actually kill. The bacteria in your biological filter — which UV deliberately does not reach — are covered in the guide to comammox and the nitrogen cycle.
