UV Sterilizer Wattage: 19 GPH Per Watt, or 6 for Ich

The 25-watt unit that is rated 475 gph and 150 gph at the same time

LiveAquaria's UV sterilization chart gives a 25-watt unit two maximum flow rates at once: 475 gph against bacteria and algae, and 150 gph against parasites. Same lamp, same body, same quartz sleeve. The only variable is what you are trying to kill, and the answer is a 3.2x cut in flow. Which is the first thing to understand about aquarium uv sterilizer wattage: the number on the box is not a capacity, it is half of a ratio.

The other half is dwell time, meaning how long a given parcel of water sits in front of the lamp. AquariumStoreDepot puts this plainly, arguing that a 25-watt TMC Vecton run at the correct flow rate outperforms a 40-watt budget unit pushed too hard. Dose is measured in µWs/cm². Slow the water and the dose climbs on hardware you already own.

Pentair's published figures for the Smart UV 25, a single 18-watt lamp, make the gap larger still: 30,000 µWs/cm² for algae and bacteria at a suggested 472 gph, against 180,000 µWs/cm² for protozoa at a suggested 79 gph. That is roughly six-fold. Every "up to 300 gallons" claim on a listing is an algae rating quoted without the qualifier.

What follows rebuilds sizing around gph per watt against a named target organism, then runs the eight units here through that criterion. If you are treating green water, most of this will not change your purchase. If you are chasing ich or velvet through a stocked system, it will change your pump, your plumbing, or both, and probably your expectations about what the unit was ever going to do.

The short version

• The pick: Aqua UV Classic, for you want a genuine plumbed inline unit at the smallest wattage in the range, for a tank around the 75-gallon mark where a 25W or 57W unit would be overkill.
• Also worth it: coospider-repta, for you run a 70–200 gallon freshwater or saltwater tank with a hang-on or canister filter chamber and want a low-wattage in-filter clarifier rather than a plumbed inline unit.
• How this was decided: published specifications and buyer reports across 8 products, compared below.

Sizing from your pump: gallons per hour per watt

Start from the pump, not the tank.

Divide your return pump's flow in gph by the lamp wattage. That single ratio, gallons per hour per watt, tells you which organisms the unit can actually touch, because dwell time past the lamp is what sets delivered dose. LiveAquaria's own selection chart, read across its two columns, gives the thresholds.

• Under 19 gph per watt: clarity and algae. LiveAquaria rates a 25-watt bulb at 475 gph against bacteria and free-floating algae, which works out to 19 gph per watt. The 40-watt figure is 940 gph (23.5), the 80-watt 1885 gph (23.6), the 130-watt 3400 gph (26.2). Call the safe working line 19.
• The 8-watt entry: clarifier only. The same chart's 8-watt entry sits at 120 gph, 15 gph per watt, and carries no parasite rating whatsoever. Small lamps are clarifiers. That is the whole of what they do.
• 6 gph per watt or slower: protozoan parasites. LiveAquaria's parasite column gives 300 gph on 40 watts (7.5), 625 gph on 80 watts (7.8), 1140 gph on 130 watts (8.8). Pentair is harsher still: the Smart UV 25's single 18-watt lamp is rated 79 gph suggested for protozoa, 4.4 gph per watt.

Two conditions attach to all of this. The volume you size against is the whole system, display plus sump rather than the display alone, and LiveAquaria advises the unit's maximum flow rate should exceed that combined gallonage. Second, the ratio assumes your pump delivers its rated flow, which head height and plumbing losses reliably prevent; the three variables that cut filter gph below the label apply equally to a return pump feeding a UV loop.

A 120-gallon reef running 360–600 gph on the return sits at 14–24 gph per watt on a 25-watt unit, which Bulk Reef Supply describes as workable for algae and bacteria but too fast for protozoa on a 25-watt unit.

Why the consensus is one source counted twice

Search "what size UV sterilizer for a 100 gallon tank" and two authorities answer. Aqua Ultraviolet puts the parasite minimum at 18 watts with flow capped at 100 gph, and prefers 25 watts at 150 gph. LiveAquaria's selection guide gives the same three numbers: 18 watts, 100 gph maximum, 25 watts at 150 gph preferable. Not similar figures. Identical ones.

That matters because two agreeing sources read as corroboration. Here they are one table, republished. Whatever assumptions sit behind those numbers (the target organism, the assumed lamp age, the assumed sleeve condition) you cannot check them by cross-referencing, because there is nothing to cross-reference against. The consensus has a single origin.

What buyers actually install

Aqua Ultraviolet states that any aquarium under 75 gallons needs an 8-watt sterilizer at 120 gph, a figure the evidence flags as contested, and which a long-time reefkeeper's account holds to be considerably undersized by today's standards. Hold that against what people actually buy. A thread on the Atlanta Reef Club cites current market convention as a 57-watt unit on a 120-gallon tank at around $500. Run the arithmetic: 8 watts across 75 gallons is 0.107 W/gal; 57 watts across 120 gallons is 0.475 W/gal. About 4.5 times the wattage per gallon, for the same stated job.

No source explains the gap. The 8-watt figure carries no parasite rating in LiveAquaria's chart, which is consistent with a clarifier-only role; why current convention sits several times higher per gallon is not documented. One hobbyist implies marketing rather than better science, but no source demonstrates it.

Sizing by goal instead

So pick by goal, not by tank size. Clearing suspended algae in freshwater under 75 gallons? The low number holds. Suppressing free-swimming parasites in a stocked marine system? Expect to pay for the lamp. Aqua Ultraviolet's own baseline already asks for twice the wattage per gallon in saltwater as in freshwater.

The same reefkeeper reports years of parasite control with 15 watts on a 55-gallon FOWLR tank, and says such sizing is today considered considerably undersized by most. Both statements are his.

Clear water false positive

Sparkling water proves your UV killed algae. It proves nothing else. Bacteria and algae die at lower UV doses than parasites, so one wattage supports very different flow rates depending on the target organism, and a unit run past its parasite flow rate can still be clearing a green tank. That gap between visible result and actual dose is the clear water false positive.

No source offers a field test, meter or observable check for verifying a unit is working at its rated output.

AquariumStoreDepot puts it bluntly: the vast majority of UV sterilizers on sale are capable only of clarification, not bacterial or parasite mitigation. The same source splits UV performance into three tiers, clarification, sterilization and heavy sterilization, with inexpensive units reaching only the first. Aqua Ultraviolet says the same thing from the other side, warning buyers outright that a small UV will not cut it and they will be disappointed with the results.

Two corollaries follow. UV controls pathogens in the water column only; it will not cure a fish that is already sick. And bulb output typically falls roughly 20–30% after a year of use, per the Humble.Fish community writeup, though no source quantifies what that decay does to a given unit's effective dose.

AquariumStoreDepot blames UV's bad reputation on exactly this: wrong unit, wrong expectation.

Product	Item Weight	Suits
coospider-repta	—	Best in-filter compact
WHITE SHARK	8.9 ounces	Best long-cord multi-use
Aqua UV Classic	2.86 kg	Best small inline
Aqua Ultraviolet AAV00021	3.08 kg	Best wiper-cleaned 25W
Aqua Ultraviolet AAV00035	—	Best discreet finish
Aqua Ultraviolet AAV00070	16 ounces	Best high-wattage value
Aqua Ultraviolet AAV00072	16 ounces	Best top-end serviceable
Aqua UV 57	0.27 g	Best plain 57W

Spec columns quote the listings verbatim. “Suits” is editorial.

Sort it in ten seconds

• coospider-repta, not for you if this matters: must be hidden inside the filter or behind an opaque plate so UV does not shine on fish.
• WHITE SHARK, not for you if this matters: needs a straight mounting run longer than 9.45 in. and an internal width over 1 in..
• Aqua UV Classic, not for you if this matters: no wiper mechanism, so the quartz sleeve has to be cleaned by opening the unit.

The eight units against the dose criteria

Eight units, three formats. Sources do point to chamber geometry and sleeve condition as factors alongside wattage: dwell time is set by chamber length and diameter, and quartz sleeves need cleaning at least every six months.

The two submerged 13-watt units

Both are internal designs, and neither publishes a flow rate, because neither controls one.

• coospider-repta 13W. The manufacturer rates it for 70–200 gallon tanks, with an 11-watt UV-C lamp emitting at 254 nm.
• WHITE SHARK OCEAN 13W. The listing claims up to 300 gallons on the same wattage.

Aquarium Store Depot makes the point directly: hang-on and internal units, the easiest to install and the bulk of the cheap end of the market, offer little dwell time. AquariumStoreDepot also says the vast majority of UV sterilizers on sale are capable only of clarification rather than bacterial or parasite mitigation.

Chamber diameter, not just watts

The Humble.Fish & Reef community writeup on sterilizer selection makes the argument the listings never do: housing diameter drives contact time, with makers running anything from 2" bodies up to 3"–5", and larger diameter meaning longer exposure and better kill at identical rated wattage. Aquarium Store Depot adds the corollary. Lowering flow only compensates up to a limit if the chamber is too short or narrow. The evidence file gives no port or housing dimensions for the Aqua Ultraviolet units, so their placement on that spread cannot be checked, and no source links chamber diameter to the 150 gph parasite figure.

What the file does not carry

The evidence file has no specifications for a 15-watt Aqua UV Classic: no tank rating, no flow figure, no sleeve-cleaning detail. None of that can be stated here. What the sources do note is that one reefkeeper controlled parasites for years with a 15-watt Aquanetics UV on a 55-gallon FOWLR system, while adding that such sizing is now widely considered considerably undersized.

There is also no listing data for the AAV00021, AAV00035 or the 57-watt units, so their bodies, finishes and specification gaps cannot be compared here. BRS does price the Aqua Ultraviolet 25-watt Classic at $449.48 and the 57-watt Classic at $691.77. Bulk Reef Supply names Aqua UV and Pentair Aquatics the two leading saltwater manufacturers, and notes Aqua UV uses a slightly less aggressive UV dose at its recommended flow rates than Pentair.

When slowing the flow beats buying watts

On a high-flow reef system, one response to a UV that is not reaching parasite dose is lower flow rather than more wattage: Bulk Reef Supply notes that slower flow through the same unit yields a higher sterilization rate, while also describing deliberate oversizing as an option. A 120-gallon saltwater tank commonly runs a return moving 360–600 gph, and Bulk Reef Supply notes that this rate is fine for algae and free-floating bacteria but far too fast for protozoa on a 25-watt unit. Bulk Reef Supply's own suggestion is deliberate oversizing, such as a 50-watt unit on a 120-gallon tank, so the parasite dose is still met at the flow the existing return pump produces. Splitting off a dedicated slower feed is not a route any source in the file documents.

Not every builder splits the flow. One 220-gallon mixed reef documented on Humble.Fish ran a 90-watt Lifegard with 100% of return flow, about 600 gph, through the unit.

Two limits keep this from being free. Aqua Ultraviolet warns that flow that is too slow heats the water, raising tank temperature and damaging the UV lamp and other life in the tank. And a single unit is tuned to one organism at a time: targeting both green water and protozoa means two sterilizers at two flow rates, not one clever compromise. Before you tee anything off, check what your return actually delivers, because three variables cut filter GPH below the label and the dose calculation inherits every one of them.

Running cost and the bulb that still lights but no longer sterilizes

Electricity is the cheap part. The evidence file contains no electricity rate or running-cost figures for UV lamps, so no annual power cost can be stated here.

Consumables are where the wattage decision actually bills you. The Humble.Fish & Reef community puts annual UV maintenance at $73 to $190 depending on wattage and brand, with an Aqua Ultraviolet 80-watt at $190 a year for two bulbs and O-rings. Quartz sleeve cost varies widely by unit: $35 on a Lifegard 55-watt against $168 on the Aqua Ultraviolet 80-watt, with a $161 ballast behind it.

Then the decay nobody meters. The same community puts the drop in bulb output at roughly 20–30% after a year of use, noting that better units build this into their advertised rating while cheaper ones do not always, and LiveAquaria's selection guide sets replacement at 9 to 12 months of continuous use. No source offers a field test or observable check for confirming a unit is still working at its rated output.

coospider-repta — Best in-filter compact

• 13-watt draw with an 11-watt UV-C lamp, rated by the maker for 70–200 gallon tanks
• Silica glass tube emitting at 254 nm, the germicidal wavelength
• But: Must be hidden inside the filter or behind an opaque plate so UV does not shine on fish
• But: Submerged-in-filter design means flow rate and contact time are whatever your filter happens to do, not a controlled figure

WHITE SHARK — Best long-cord multi-use

• 9.84 ft power cord reaches outlets well away from ponds, sumps and outdoor water features
• 13 watts rated for water volumes up to 300 gallons
• But: Needs a straight mounting run longer than 9.45 in. and an internal width over 1 in., plus 0.4–0.8 in. clearance for the cable bend
• But: Submerged rather than inline, so dwell time depends entirely on your pump

Aqua UV Classic — Best small inline

• Rated at 75 gallons, so wattage matches a mid-size tank instead of forcing you up the range
• Inline clarifier/sterilizer format gives a defined flow path past the lamp
• But: No wiper mechanism, so the quartz sleeve has to be cleaned by opening the unit
• But: 15 watts caps how far you can push flow before contact time drops below sterilizing dose

Aqua Ultraviolet AAV00021 — Best wiper-cleaned 25W

• Wiper knob cleans the quartz sleeve by pulling, no disassembly needed
• 2-inch inlet and outlet suit large-diameter pond and aquarium plumbing
• But: Listing quotes both 150 and 1200 gallons, which only makes sense as clarifier versus sterilizer dosing — the sterilizing figure is the low one
• But: 25 watts on 2-inch pipe means you must throttle flow to hold contact time

Aqua Ultraviolet AAV00035 — Best discreet finish

• Wiper feature on a 25-watt body for maintaining lamp output between service intervals
• 2-inch ports match large-bore plumbing
• But: Listing carries no specs at all — no weight, no volume rating, no flow guidance
• But: Turnover and contact time must be worked out from the manufacturer's own tables, not the listing

Aqua Ultraviolet AAV00070 — Best high-wattage value

• 57 watts gives the most headroom of any unit listed, so you can run higher flow and still hold dose
• 2-inch ports for large systems
• But: No wiper, so output falls off as the quartz sleeve films over until you open it
• But: No gallon rating on the listing, leaving sizing to the manufacturer's flow charts

Aqua Ultraviolet AAV00072 — Best top-end serviceable

• 57-watt lamp with a wiper — the only combination here that pairs high output with in-place sleeve cleaning
• 2-inch inlet and outlet for high-flow plumbing
• But: Highest wattage means highest running cost, unjustified below a few hundred gallons
• But: Listing includes contradictory generic fields such as 'Remote' control method and 'Is Waterproof: false'

Aqua UV 57 — Best plain 57W

• 57-watt output on 2-inch ports, the top of this wattage range
• Plastic body with AC/DC power supply for straightforward inline mounting
• But: Listing gives no gallon rating, no wiper and an implausible 0.27 g weight, so almost nothing is verifiable from it
• But: Without a wiper, sleeve fouling quietly cuts the delivered dose

Which unit for which system

Back to the 120-gallon reef.

The evidence file gives no dose figures for Cryptocaryon at a defensible confidence level. The one research figure cited, 280,000–800,000 µW/cm² from University of Florida extension work, contradicts every manufacturer number, and the same source calls that literature poorly controlled or anecdotal. It also carries no specifications for a 57-watt wiper unit. What the sources do say is that a UV must never be sized below the system volume, and that quartz sleeves need cleaning at least every six months.

If your target is green water rather than parasites, the wattage argument collapses. Bulk Reef Supply treats manufacturer tank ratings as a floor, not a target, and LiveAquaria states outright that bigger is better. For clarity alone, one Atlanta Reef Club member runs UV only during the display's 10–12 hour photoperiod, purely for water clarity rather than disease control. Whether part-time operation works is contested: Bulk Reef Supply suggests running a UV 12 hours on, 12 off is almost pointless, while another reefkeeper recommends timers to spare copepods. No source measures what a duty cycle does to pathogen population control.

Matching the eight to a pump and a target:

The evidence file contains no specifications, ratings, run-time guidance, cord lengths or plumbing details for any of these eight units, so no unit-by-unit matching can be supported here.

Bulk Reef Supply's harder point stands: one unit tunes to one sterilization level. Two targets means two sterilizers at two flow rates.

What this comparison rests on

This comparison rests on manufacturer dose tables, retailer sizing guides and forum-documented builds. Nothing here was measured on a bench, and no source in the file publishes a formula converting wattage, flow rate and chamber dimensions into a delivered dose. Switch the lamp off during chelated copper treatment, because UV breaks the chelate and dumps ionic copper, and off entirely while the tank cycles.

What to watch, in two service intervals: quartz sleeve cleaning at least every six months, and bulb replacement at 9 to 12 months of continuous use.

Which one, then?

Aqua UV Classic, best for you want a genuine plumbed inline unit at the smallest wattage in the range, for a tank around the 75-gallon mark where a 25W or 57W unit would be overkill.

If your case is different:

• For you run a 70–200 gallon freshwater or saltwater tank with a hang-on or canister filter chamber and want a low-wattage in-filter clarifier rather than a plumbed inline unit: coospider-repta.
• For you need one low-wattage clarifier that can move between a sump, a koi pond, a spa or a cold plunge, where the power outlet is nowhere near the water: WHITE SHARK.
• For you have a large system on 2-inch plumbing and want a 25-watt inline unit whose sleeve you can clean without breaking down the pipework: Aqua Ultraviolet AAV00021.

The Aqua UV Classic’s weak spot: no wiper mechanism, so the quartz sleeve has to be cleaned by opening the unit