UV Sterilizers in Aquariums: What They Do and Don't
What a UV sterilizer really does — clearing green water and killing waterborne microbes — what it cannot do, and how flow rate, contact time and wattage decide whether it works.
What a UV sterilizer does
A UV sterilizer (UV clarifier) pumps aquarium water through a sealed chamber past an ultraviolet-C lamp. The UV-C light damages the DNA of micro-organisms carried through it, so they can no longer reproduce or are killed outright. In practice this clears 'green water' caused by free-floating single-celled algae, and reduces waterborne bacteria and free-swimming parasites and dinoflagellates in the water column.
What it does NOT do
- It will not cure a fish that already shows ich/whitespot on its body — attached or embedded parasites never pass through the chamber.
- It will not remove algae or cyanobacteria growing on the substrate, rocks or glass; only organisms suspended in the water are affected.
- It only treats the water that actually flows through it, so it is population control, not a guaranteed cure.
- It does not replace mechanical or biological filtration — it is an add-on, not a substitute.
Flow rate, contact time and wattage
The decisive factor is contact time — how long each drop of water is exposed to the lamp. The faster the water flows, the lower the dose, so UV units are deliberately run at modest flow. A common recommendation is to pass roughly 3 to 4.5 times the tank volume per hour through the unit, with about 2-4 watts of UV per gallon; clearing green algae needs only a light dose, while killing hardy targets such as whitespot demands a far higher dose that is often impractical at normal flow rates. What matters is genuine UV-C output, not just the lamp's total wattage.
Sources: reefbuilders.com , reefbuilders.com