UV kills only the free-swimming theront stage of Cryptocaryon irritans. Cysts on the sand and trophonts on the fish aren't reached by water passing through the unit. UV reduces the infection rate but cannot eradicate ich.

Will UV kill beneficial bacteria?

No. Beneficial nitrifying bacteria are attached to surfaces (rock, sand, sponge media), not free-floating. UV only kills what passes through the chamber.

Should I run UV 24/7?

Yes if you have algae blooms or want continuous parasite reduction. Many reefers run it only during ich outbreaks or new-fish acclimation periods.

Guide · 4 min read · 650 words

Sizing a UV sterilizer for a reef tank

UV sterilizer sizing is the most-misunderstood spec in the reef hobby. People shop by "rated tank size" on the box. The actual variable is flow rate through the chamber.

The real spec is flow rate

UV sterilizer manufacturers print "rated for 100-gallon tank" on the box. This is meaningless. The variable that matters is flow rate through the chamber.

A unit with a 25 W bulb passing 12 gallons per hour delivers ~15× the UV exposure of the same unit at 400 GPH. The bulb intensity hasn't changed — the water is just spending more time being irradiated.

What UV actually kills

Bacteria (free-floating): Killed at 30,000 µW·s/cm². Easy.
Algae spores and dinoflagellates: 22,000–60,000 µW·s/cm². UV is highly effective against dino blooms.
Ich theronts (free-swimming): 100,000 µW·s/cm². The floor for "parasitic dose."
Ich cysts and tomonts on substrate: Not killed — they don't pass through the chamber.

Wattage targets by use case

Algae bloom control: 1 GPH per watt, 1 W per 5 gallons.
Bacterial sterilization: 1.5 GPH per watt, 1 W per 5 gallons.
Parasitic dose (ich, velvet): 0.5 GPH per watt, 1 W per 3 gallons. Slow flow + bigger bulb.

Bulb life and replacement

UVC output drops linearly with run time. Most bulbs maintain 60% output through ~9 months and drop quickly after. Replace annually regardless of whether the bulb still lights — the visible blue glow doesn't mean UVC output is still effective.

What to buy

Pentair / Aquanetics, Aqua UV, and Coralife are the established brands. For reef tanks 75 gal and up: 25 W minimum, throttled with a ball valve to target flow. For nano (20 gal): the Green Killing Machine 9 W internal works for algae and basic bacterial reduction.

Algae identification — UV works against some types but not all.

Equipment mentioned in this guide

Green Killing Machine for nano internals. Pentair Aquanetics inline UV for larger tanks. Annual bulb replacement \u2014 the visible blue glow doesn't mean UVC output is still effective.

AA Aquarium Green Killing Machine 9W UV

Internal UV. Best for nano tanks + algae control.

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Pentair Aquanetics inline UV sterilizer

External UV. Sized 15W–80W for tank parasites.