UV sterilizers kill free-floating bacteria, parasites (in the free-swimming theront stage), and algae spores that pass through the bulb chamber. Done correctly, UV reduces ich outbreaks, clears green water, and lowers free-floating bacteria counts. Done incorrectly (under-sized, wrong flow rate, dirty sleeve), it does almost nothing.
Algae control: 1 watt per 10 gallons. Bacterial reduction: 2 watts per 10 gallons. Parasite kill (ich, velvet): 4 watts per 10 gallons + slow flow. A 100-gallon reef targeting parasite kill needs a 40-watt UV.
UV kills based on contact time. Too fast = pathogens pass through alive. Manufacturer specs typically list "algae rate" (faster) and "parasite rate" (slower, often 50% of algae rate). For a 40-watt UV with 30 GPH parasite rate, run a 30 GPH dedicated pump - not your 1200 GPH return.
Pentair Aquatic Eco-Systems Smart UV ($300-500): commercial-grade, the standard for serious reefs. Aqua UV ($150-400): solid mid-tier, available in 8-114 watt sizes. Coralife Turbo Twist ($80-180): budget option, fine for small tanks under 50 gallons. Avoid: green-light UV bulb units sold without UVC certification.
UV-C bulbs lose 50% intensity at 9-12 months even though they still emit visible blue light. Replace bulbs every 12 months on a calendar reminder. The most common UV failure is "the bulb still glows but isn't killing anything."
A film of biological gunk on the inside of the quartz sleeve blocks UV-C from reaching the water. Pull the sleeve every 3-4 months, wipe with vinegar, reinstall. Skipping this step kills 60-80% of UV effectiveness.
UV destroys some additives (vitamins, amino acids dosed for SPS color). If running heavy supplementation, schedule UV to run 18 hours per day with a 6-hour gap right after dosing. UV also breaks ozone if you run an ozonizer - run them in series, not parallel.