How UV Sterilization Works and What It Kills
Ultraviolet light at 254nm wavelength penetrates the cell walls of microorganisms and damages their DNA, preventing reproduction. The critical variable is UV dose, calculated as irradiance (watts per square centimeter) multiplied by exposure time (seconds). Higher flow rates reduce exposure time and lower the effective dose — a 9-watt UV unit running at 400 GPH delivers a fraction of the lethal dose for Ich compared to the same unit at 100 GPH. This is why manufacturer watt ratings are nearly meaningless without knowing the flow rate for which they are designed.
Green water (suspended Chlorella algae) requires the lowest UV dose — roughly 20,000–40,000 µW·s/cm². Most correctly sized units running at recommended flow rates eliminate green water within 3–5 days. Free-floating bacteria require a higher dose of approximately 40,000–65,000 µW·s/cm². Protozoan parasites like Ich require 90,000–180,000 µW·s/cm² to neutralize — achievable only at very low flow rates, typically 25–50 GPH per watt of UV output. This is why you need a larger, dedicated slow-flow UV unit to combat active parasite outbreaks rather than relying on the filter-turnover rate that works for algae control.
UV sterilizers do not kill beneficial filter bacteria because those bacteria are attached to surfaces in your filter media and tank substrate — they never pass through the UV chamber. This is a common misconception that prevents many fish keepers from using UV when it would benefit them significantly. The free-floating bacterial fraction in your water column is predominantly pathogenic or transient — it does not contribute to the nitrogen cycle.
- ✦Size your UV sterilizer at 1 watt per 3–4 gallons for algae control, and 1 watt per 1–2 gallons for parasite control — never rely on the maximum gallon rating on the box.
- ✦Replace UV bulbs every 6–12 months regardless of whether they still glow — UV output degrades to ineffective levels long before the visible light disappears.
- ✦Keep your UV unit's quartz sleeve clean: mineral deposits and biofilm on the sleeve block UV transmission and can reduce effective output by 50–80% even with a new bulb inside.
Installing and Sizing a UV Sterilizer Correctly
Inline UV installation — connecting the unit in series with your canister filter return line — is the most efficient configuration. Water flows through under pump pressure at a controllable rate, and the entire water volume is treated every time the pump runs. For a 55-gallon community tank targeting algae control, a 9-watt inline unit (Green Killing Machine, AA Aquarium, or Aqua UV) at 200–250 GPH provides adequate exposure at a reasonable cost. For the same tank targeting Ich in a quarantine scenario, drop flow to 80–100 GPH through the same unit or upgrade to an 18-watt unit.
Hang-on-back and submersible UV units perform adequately for green water control in smaller tanks but are inefficient for disease control because water can bypass the exposure chamber when flow rates exceed the housing's capacity. If you are fighting a persistent health problem, always choose inline installation. Ensure all UV plumbing connections are tight before running — even small UV exposure is an eye and skin hazard; never look directly into the light when the unit is running.
- ✦Run UV continuously for established community tanks; for reef tanks, turn UV off during coral feeding periods (2–3 hours) so iodide-rich trace elements are not oxidized before corals can absorb them.
- ✦If you use live phytoplankton in a reef or shrimp tank, bypass the UV during dosing — the UV will destroy phyto before it can benefit your livestock.
- ✦Check flow direction arrows on inline UV housings before installation — reversed flow reduces dwell time inside the exposure chamber by up to 40% compared to correct orientation.