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UV STERILIZER

Flow rate through a UV for algae, bacteria, or parasites. The three targets need very different speeds.

Flow rated for algae, GPH
The manufacturer figure at 30,000 µW·s/cm². Most publish this as their headline number.
What you are treating
Custom dose, µW·s/cm²
Flow you actually run, GPH
System volume
Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Flow for protozoa180,000 µW·s/cm²
200GPH maximum
At 400 GPH you get
90,000µW·s/cm²
Below the target. Slow the flow to 200 GPH.
Turnover through the UV
5.33× per hour
Water has to pass through to be treated. Very slow flow gives a high dose but treats little of the system.
Full pass of the system
0.2hours
How long for a volume equal to the whole system to pass through once.
Replace the bulb
yearly
Output falls well before the lamp stops lighting. A glowing bulb past twelve months is doing a fraction of its rated work.
TargetDoseMax flow
Algae, green water30,0001,200 GPH
Bacteria45,000800 GPH
Protozoa, ich and velvet180,000200 GPH

UV dose is irradiance multiplied by contact time, and contact time is inversely proportional to flow. So halving the flow doubles the dose. Manufacturers publish one flow per dose level, and every other level follows from that by simple proportion.

What UV cannot doA UV only kills what passes through it. Ich spends most of its life cycle on the fish and in cysts on the rock and glass, where no UV reaches. It reduces the free swimming stage and slows an outbreak, and it does not cure ich. Quarantine does.
MethodFormula and sources

flow = 1,200 GPH × 30,000 ÷ 180,000 = 200 GPH

Ultraviolet dose is measured in microwatt seconds per square centimetre, and it is irradiance times how long the water sits in the chamber. Since residence time is inversely proportional to flow for a fixed chamber, dose and flow multiply to a constant.

That makes the whole thing one proportion. Manufacturers publish a flow rate at 30,000 µW·s/cm², and every other dose follows: flow = base × 30,000 ÷ target dose. Checked against Aqua Ultraviolet's own published columns, this reproduces their figures exactly.

The three targets are far apart. Free floating algae goes at 30,000, bacteria at 45,000, and protozoa like the free swimming stage of ich need 180,000. That last one is six times the dose, which means one sixth of the flow, and it is why a UV sized for green water does nothing against parasites.

Slower is not simply better, because a UV only treats water that passes through it. Running at a sixth of the flow gives the dose but takes six times as long to process the system. Sizing means picking a unit large enough to deliver the dose at a useful flow, not throttling a small one to nothing.

Bulbs age invisibly. UV output drops substantially over a year while the lamp still glows, so replace on schedule rather than on appearance. The quartz sleeve also needs cleaning, since a film of coralline blocks the light before it reaches the water.

A UV kills what flows through it and nothing else. Ich lives most of its cycle attached to fish or encysted on rock and glass, out of reach, so a UV slows an outbreak rather than curing one. Quarantine is what cures it.

UV also breaks down some trace elements and medications. Turn it off during treatments and when dosing anything light sensitive.

Sources: Pentair Smart UV manual (PDF) · Aqua Ultraviolet Classic manual (PDF)

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