How a UV sterilizer works
A UV sterilizer pumps water past an ultraviolet lamp. The light damages the DNA of free-floating algae cells, bacteria and parasites so they cannot reproduce. Only organisms in the water column pass the lamp: algae on the glass, parasites attached to fish and the beneficial bacteria living in your filter are not affected (LiveAquaria).
What matters is the dose: UV intensity × the time water spends near the lamp, reduced by anything that absorbs UV. Manufacturers rate algae and bacteria at about 30,000 µW·s/cm² (30 mJ/cm²). Parasite ratings are much higher: Aqua Ultraviolet uses 90,000 µW·s/cm² and Pentair 180,000 µW·s/cm² for protozoa. That is why the same unit is rated for a far lower flow when parasites are the target.
UV clarifier vs UV sterilizer
The hardware is the same; the difference is the dose. Run at a lower dose and higher flow, a unit acts as a clarifier: it kills enough floating algae to clear green water. Run at a higher dose and lower flow, it acts as a sterilizer: it also controls bacteria and, at the highest ratings, free-swimming parasites.
Aqua Ultraviolet, for example, rates its Classic 25 W at 1,200 GPH as a freshwater sterilizer (30,000 µW·s/cm²), 400 GPH for salt water and parasites (90,000 µW·s/cm²), and up to 2,000 GPH as a clarifier for a shaded, planted pond of 4,000 gallons.
Why wattage alone is not enough
Two units with the same lamp wattage can deliver very different doses because of chamber size, lamp output, reflectors and water path. At 18 W, Pentair’s SMART UV Lite is rated to 4 GPM (240 GPH) for algae, Coralife’s Turbo-Twist 6x to 240 GPH, and EHEIM’s reeflexUV 1500 to 2,000 L/h (528 GPH). That is why the calculator uses each model’s published flow for your goal and shows wattage only as a labelled typical estimate.
Aqua Ultraviolet’s dose tables also show that flow × dose stays constant for a unit: the Classic 25 W is rated 1,200 GPH at 30,000, 600 GPH at 60,000 and 400 GPH at 90,000 µW·s/cm². The calculator uses this to scale a published rating to a custom target dose, and marks the result as scaled.
Worked examples
| Situation | Flow through the UV | What the calculator says |
|---|---|---|
| 55 US gal (208 L) freshwater, green water, new unit | 165–248 GPH (3–4.5×) | Typical size 18 W. Rated options include Coralife Turbo-Twist 6x (240 GPH for algae) and EHEIM reeflexUV 800 (1,200 L/h). |
| Same tank, Coralife 6x on a 400 GPH pump | 400 GPH (7.3×) | Too fast: 67% above the 240 GPH algae rating. Fit a valve or a smaller pump. |
| 75 US gal (284 L) marine, parasites, new unit | 38–112 GPH (0.5–1.5×) | Typical size 25 W. Aqua Ultraviolet Classic 25 W is rated 400 GPH at 90,000 µW·s/cm². |
| Same tank, Classic 25 W at 600 GPH | 600 GPH | Too fast: 50% above its parasite rating, roughly 60 instead of 90 mJ/cm² per pass. |
| 2,000 US gal shaded koi pond, green water | 1,000–2,000 GPH (0.5–1×) | Aqua Ultraviolet clarifier ratings start at 15 W (up to 1,800 GPH for 2,000 gal). |
What UV can and cannot help with
- Helps: green water (floating algae), bacterial cloudiness, and free-swimming stages of parasites, when the flow suits the unit’s rating.
- Does not help: algae on glass, plants or decor (including hair algae and black beard algae), yellow or tea-coloured water from tannins, ammonia or nitrite problems, or parasites already on a fish.
- Does not fix the cause. Excess light and nutrients bring algae back once the UV is off. Tropica suggests a few days of darkness and frequent water changes for green water.
- Supports, never replaces, disease treatment. Diagnose the problem, quarantine new fish and use a proven treatment.
- Lamps fade. Ratings assume replacement at the end of lamp life: about 8,000 hours (Coralife), 9,000 hours (Pentair) or 14 months (Aqua Ultraviolet). Keep the quartz sleeve clean.
- Reef tanks: Aqua Ultraviolet recommends a 30,000–45,000 µW·s/cm² rating; higher doses also kill plankton that corals feed on.
- Turn it off during some treatments. UV can break down certain medicines; follow the product label.
